The following is an excerpt of portions of the prospectus contained in the Form S-4 registration statement (File No. 333-218885) as declared effective by the Securities and Exchange Commission on

The following is an excerpt of portions of the prospectus contained in the Form S-4 registration statement (File No. 333-218885) as declared effective by the Securities and Exchange Commission on July 13, 2017. Such information is as of July 13, 2017 (unless an earlier date is indicated).

Synlogic is pioneering the development of Synthetic Biotic medicines: a novel class of living medicines intended to treat a broad

range of human diseases, ranging from genetic and acquired metabolic disorders to inflammation and cancer. Synthetic Biotic medicines are generated from Synlogic s proprietary drug discovery and development platform. Synlogic applies the

principles and tools of synthetic biology to engineer beneficial probiotic bacteria to perform or deliver critical therapeutic functions, compensating for missing or damaged pathways in patients with these serious diseases. As living medicines,

Synthetic Biotic medicines are designed to sense a local disease context within a patient s body and to respond by metabolizing toxic substances or delivering combinations of therapeutic factors.

Synlogic s initial focus is on metabolic diseases with potential to be corrected following oral delivery of a living medicine to the gut.

This includes a group of rare genetic diseases called inborn errors of metabolism ( IEMs ), as well as acquired metabolic diseases caused by organ dysfunction:

In patients with these diseases, byproducts of failed metabolism can accumulate to toxic levels and cause serious health consequences

throughout the body. Synthetic Biotic medicines are designed as oral therapies to act in the gut to convert toxic metabolites into non-toxic byproducts and, as a result, reduce toxic metabolite levels in the

systemic circulation and tissues. Synthetic Biotic medicines are engineered to clear toxic metabolites specific to each metabolic disease and have the potential to provide meaningful benefits to patients suffering from these debilitating conditions.

Synlogic initiated a Phase 1 clinical trial for its lead Synthetic Biotic program, SYNB1020, in June 2017. SYNB1020 is in development as

an oral treatment for patients with hyperammonemia. In patients with hyperammonemia, ammonia accumulates in the body and becomes toxic leading to neurocognitive crisis and risk of long-term cognitive or behavioral impairment, coma, or death.

Hyperammonemic conditions include a group of IEMs known as Urea Cycle Disorders ( UCD ), and hepatic encephalopathy ( HE ) in liver disease patients. SYNB1020 is designed to remove excess ammonia from the gut by converting it

into the beneficial amino acid arginine, with potential to result in lowered ammonia levels in the blood. Synlogic s second program, SYNB1618, is an oral therapy intended for the treatment of phenylketonuria ( PKU ), an IEM in which

the amino acid phenylalanine accumulates as a result of genetic defects, becoming toxic to the brain and leading to neurological dysfunction. SYNB1618 is designed to have activity in the gut of patients to reduce excess phenylalanine to result in

normalization of levels in the blood and tissues. Synlogic is planning to initiate a Phase 1 clinical trial for SYNB1618 in the first half of 2018. Synlogic s earlier metabolic disease pipeline includes discovery-stage product candidates for

additional IEMs, such as maple syrup urine disease ( MSUD ), isovaleric acidemia ( IVA ) and organic acidemias.

Synlogic s platform also has the potential to generate clinically meaningful therapies for patients affected by immune-mediated diseases

and cancer. Synthetic Biotic medicines are designed to locally deliver combinations of complementary therapeutics to treat these complex disease states. Synlogic s portfolio of immuno-oncology programs is designed to deliver a combination of

activities to modify the tumor microenvironment, activate the immune system and result in tumor reduction. In addition, Synlogic has established a strategic collaboration with the integrated pharmaceutical company AbbVie to develop Synthetic

Biotic-based treatments for inflammatory bowel disease ( IBD ) such as Crohn s disease and ulcerative colitis. While Synlogic intends to develop and commercialize therapeutic candidates for the treatment of IEMs on its own, Synlogic

may consider entering additional strategic partnerships in the future to maximize the value of Synlogic s programs and its Synthetic Biotic platform.

To progress its pipeline, Synlogic collaborates with key disease experts who have developed robust models of relevant diseases to guide

selection of Synlogic s development candidates and to inform its translational medicine strategy. Synlogic focuses on indications with clear biomarkers associated with disease progression that enable straightforward, early and ongoing

assessment of potential clinical benefit throughout the development process. Synlogic s collaboration and intellectual property strategies additionally focus on building or leveraging existing third-party expertise in therapeutic research, pre-clinical and clinical development, regulatory affairs, manufacturing and commercialization, while also enhancing Synlogic s industry-leading position in synthetic biology and metabolic engineering.

Synlogic has assembled a management team of seasoned biopharmaceutical executives with extensive, relevant experience at leading

pharmaceutical companies such as Pfizer Inc. ( Pfizer ), GlaxoSmithKline, Biogen, Inc. ( Biogen ), AstraZeneca, Millennium Pharamceuticals, Inc. ( Millennium Pharmaceuticals ) (now Takeda Pharmaceutical Company Limited)

and MedImmune, as well as the National Institute of Health. Synlogic is supported by the Synlogic Board of Directors and the Synlogic scientific advisory board, each of which offer complementary experience in drug discovery and development, as well

as expertise in building public companies, management, and business development. Synlogic s founding science came from the labs of Professors James Collins and Timothy Lu from the Massachusetts Institute of Technology ( MIT ), who

remain highly engaged in guiding development and application of Synlogic s platform.

Synlogic s pipeline of programs is shown below.

As Synlogic advances its lead programs, Synlogic continues to learn and improve the flexibility,

manufacturability and translatability of its Synthetic Biotic platform, which will inform all future portfolio programs. Consequently, Synlogic believes it has a robust engine for building a sustainable pipeline of novel, living medicines across a

range of diseases. Through the strength of Synlogic s internal team and network of partners, Synlogic believes it can deliver on the promise of Synthetic Biotic medicines to improve the lives of patients with significant unmet medical needs.

Synlogic s goal is to use its Synthetic Biotic platform to design, develop and commercialize living medicines to transform the lives of

patients for whom conventional treatment approaches are either not available or have limited efficacy and safety. To achieve its goal, Synlogic is pursuing the following key strategies:

Rapidly Advance Clinical Development of the SYNB1020 Hyperammonemia Program. Synlogic s lead Synthetic Biotic program is

for the treatment of hyperammonemic conditions such as UCD and HE. SYNB1020 is an oral therapy designed to deliver a complementary metabolic pathway in the gut with the intended consequence of removing excess ammonia in the blood. SYNB1020 has

received orphan drug designation and in June 2017 was granted Fast Track designation for UCD from the FDA. Synlogic initiated its first Phase 1 clinical trial to assess safety, tolerability and pharmacokinetics in healthy volunteers in June 2017.

Assuming success in the Phase 1 clinical trial, Synlogic plans to initiate an HE study to better understand safety, tolerability and therapeutic potential of SYNB1020. Synlogic expects to start the study in the first half of 2018 and to have topline

data by the end of 2018. Similarly, based on the results of the Phase 1 clinical trial, Synlogic expects to begin a clinical trial in UCD by mid-2018 with data expected in the first half of 2019.

Complete IND-Enabling Activities to Advance SYNB1618 into Clinical Development.

Synlogic s second IEM program is an oral therapy for PKU. SYNB1618 is designed to act from the gut to convert excess phenylalanine to non-toxic metabolites and thereby prevent phenylalanine from

accumulating in the blood, becoming toxic and leading to neurological dysfunction. Synlogic expects to initiate a Phase 1 trial for this candidate in the first half of 2018. The Phase 1 design will include healthy volunteers, as well as an adult

patient cohort, to assess safety, tolerability and pharmacodynamics. Synlogic expects to have final results from the healthy volunteer study, including insights from a mechanistic biomarker, by the end of 2018 and insights regarding therapeutic

potential by the first half of 2019.

Expand Synlogic s Pipeline by Targeting

Additional Rare Genetic Metabolic Diseases. Synlogic plans to continue to leverage its expertise from its lead programs to accelerate development of Synlogic s pipeline of clinical candidates for IEMs. For example, Synlogic s

portfolio includes two additional discovery-stage Synthetic Biotic programs in lead optimization, including one for MSUD/IVA and the other for propionic acidemia ( PA )/methylmalonic acidemia ( MMA ), organic acidemias with high

unmet need for which there are biomarkers that Synlogic believes can guide efficient product development programs.

Value of the Synthetic Biotic Platform in Broader Metabolic and Inflammatory Diseases and in Immuno-Oncology Leveraging Strategic Partnerships. Synlogic s Synthetic Biotic platform and product discovery and development

capabilities offer the potential to generate multiple clinically meaningful treatments for a broad set of metabolic and inflammatory diseases as well as cancer. For these indications, there is opportunity to reset a metabolic or immune dysfunction

with a lower risk of systemic toxicity than other modalities. To achieve this, oral Synthetic Biotic medicines may be designed to deliver a combination of mechanisms following oral administration for activity in the gut or intra-tumoral injection.

For example, Synlogic is establishing a discovery-stage immuno-oncology portfolio.

Synlogic expects to continue to explore strategic partnerships that would leverage the

complementary capabilities of its partners to develop Synthetic Biotic medicines for these broader groups of patients in need. Synlogic s current partnership with AbbVie is focused on the discovery and development of Synthetic Biotic-based

therapies for the treatment of IBD, and in June 2017 Synlogic announced its first milestone for this program. While Synlogic intends to develop and commercialize its programs for IEMs, Synlogic may consider entering into additional strategic

partnerships to maximize the value of its Synthetic Biotic platform in these more common indications.

Expand the Synthetic Biotic

Platform to Lead in the Discovery and Development of Additional Living Medicines and Enabling Technologies. Synlogic intends to advance in the field of living medicines by continuing to innovate and broaden the potential of its Synthetic

Biotic platform to deliver clinically meaningful benefits for patients. Synlogic plans to build on its expertise in design, optimization and manufacturing to further develop the Synthetic Biotic platform as a reproducible and scalable engine for

generating a pipeline of product candidates that address a broad range of diseases.

Protect and Leverage Synlogic s

Intellectual Property Portfolio and Patents. Synlogic believes that it has a broad intellectual property portfolio that includes patents and patent applications relevant to the engineering, development, manufacturing and formulation of human

therapeutic products based on synthetic biology and the metabolic engineering of probiotics. Synlogic intends to continue to protect and leverage its intellectual property assets by maintenance and expansion of its worldwide portfolio of

intellectual property, including through the pursuit of composition of matter and other intellectual property directed to its Synthetic Biotic programs and its technology platform.

Synlogic s Focus: Living Medicines

Synlogic is developing and advancing a novel approach to creating living medicines therapeutics designed to sense a local disease context

within a patient s body and to respond by metabolizing toxic substances or delivering combinations of therapeutic factors. Synlogic applies the tools and principles of synthetic biology to engineer beneficial probiotic bacteria to perform or

deliver critical therapeutic functions, compensating for missing or damaged pathways in patients with metabolic diseases, inflammation and cancer.

Synlogic believes living medicines have unique advantages as potential therapeutics. Living biologic cells can carry out functions that cannot

be performed by many conventional drug treatments, such as small molecules or antibodies. While many conventional treatments can address one molecular dysfunction, living medicines can compensate for the dysfunction of entire processes or pathways

missing in disease and required for health. By contrast to conventional therapeutics that engage a single target, living medicines can be designed to dynamically sense diseased environments and respond with a programmed and combinatorial effect.

Moreover, a living medicine can also function catalytically, as a single living cell can carry out multiple cycles of the intended therapeutic activity during its time in the patient.

There is opportunity to expand the impact that previous cell therapies have had by applying the well-established tools of synthetic biology to

probiotic bacteria, converting them into efficient therapeutic engines. Probiotic bacteria are non-pathogenic bacteria isolated from the human microbiota widely used as supplements believed to provide health

benefits. To confer a therapeutic effect, Synlogic leverages basic biological properties of bacteria to develop engineered probiotics. Bacteria have evolved over several billion years to adapt, survive, and carry out active metabolism in many

different environments. They are also amenable to genetic manipulation. Synlogic s intention is to lead in the discovery and development of Synthetic Biotic therapies as safe living medicines capable of robust and precise pathway

complementation and therapeutic benefit.

Leveraging Synthetic Biology and Metabolic Engineering of Probiotic Bacteria to Produce Living Medicines

Synlogic s proprietary Synthetic Biotic discovery and development platform combines synthetic biology and metabolic

engineering to re-design the genetic circuitry of beneficial probiotic bacteria and generate living medicines.

Synthetic biology is

an emerging and rapidly-evolving discipline that applies engineering principles to biological systems to enable rational, design-based control of cellular function for a specific purpose. Biological systems are governed by DNA sequences, or genes,

that code for the production and regulation of proteins, metabolites and other molecules. The regulation of the function of proteins occurs via complex biochemical and cellular reactions working through intricate signaling pathways. Synthetic

biology allows manipulation of these pathways to direct a desired therapeutic outcome. While efforts have been made to apply these principles across industries, Synlogic believes it is a leader in deploying synthetic biology for the treatment of

Synlogic scientists genetically engineer a beneficial probiotic bacterium with wiring or biological circuits

to direct cellular biological processes in a manner analogous to designing electrical circuits. The critical parts of an engineered Synthetic Biotic medicine include (1) the chassis, or probiotic bacterium, (2) the effector module, which

is a gene or pathway encoding the core biological activity that provides the therapeutic function, and (3) tunable switches to precisely determine the circumstances under which the effector module will be activated, as well as the strength,

performance and output of the effectors themselves. Synlogic aims to precisely control the amount, location and activity of its Synthetic Biotic medicines to address a broad range of disease.

Schematic of the Synthetic Biotic Platform Components: Chassis, Effector, Switch

Metabolic Engineering of Probiotic Bacteria

(1) The Chassis: Synlogic s Synthetic Biotic platform employs well-characterized bacteria used as probiotics to serve as the

chassis upon which Synlogic builds its living medicines. Synlogic s initial programs use E. coli Nissle, which is one of many non pathogenic strains isolated from the human microbiota. E. coli Nissle has been used as a

probiotic bacterial supplement for the last 20 years to promote gut health. E. coli Nissle is a non-colonizing probiotic in that it has recently been shown in the clinic to be rapidly cleared from most

individuals with no significant safety issues. Synlogic believes E. coli Nissle s widespread use as a probiotic is evidence of its utility as a safe background chassis to apply synthetic biology to confer a therapeutic effect.

(2) Building the Effector Module: E. coli Nissle s metabolic systems are well-understood and extremely adaptable, making it

an excellent organism for introducing new or enhanced activities to treat human disease. The highly flexible nature of its genetic and metabolic machinery provides a robust cellular context into which genetic information encoding proteins and

pathways to correct for disease can be introduced with high efficiency and little or no damage to the fitness of the bacterium. This provides the potential for excellent reproducibility, stability, and activity during manufacturing. Moreover, the

advanced nature of the synthetic biology toolkit available for E. coli Nissle enables the rapid iterative design, assembly, and testing of prototype product candidates and remains unique among other bacterial and cellular engineering