– IND Clearance of pheEDIT Study to Evaluate a One-Time Dose of Investigational HMI-103 Incorporating a Novel Nuclease-Free Gene Editing Approach – – Company Provides Update on Enrollment in Ongoing pheNIX Trial of HMI-102 Gene Therapy for Adults with PKU – BEDFORD, Mass., Oct. 12, 2021 (GLOBE NEWSWIRE) — Homology Medicines, Inc. (Nasdaq: FIXX), a genetic medicines company, announced today the pheEDIT Phase 1 clinical trial for HMI-103, a one-time, in vivo product candidate that utilizes a gene editing approach for phenylketonuria (PKU), based on the Investigational New Drug Application (IND) clearance from the U.S. Food and Drug Administration (FDA). HMI-103 will be the world’s first gene editing candidate for PKU to enter clinical trials from Homology’s dual gene therapy and gene editing technology platform, and with the launch of pheEDIT Homology moves closer to its goal of offering solutions for both adults and pediatric patients with PKU. “Today’s milestone is the culmination of our team’s tireless work to translate our gene editing technology from an academic discovery into a clinical program for people with PKU,” said Albert Seymour, Ph.D., Chief Scientific Officer of Homology Medicines. “Our positive preclinical data in the PKU model demonstrated phenotypic correction, and the precision of HMI-103 genome integration was confirmed in a humanized liver model, which showed no evidence of off-target mutations or unwanted on-target changes to the genome. These nonclinical data give us great confidence in initiating our Phase 1 pheEDIT trial, and we look forward to continuing to work with the PKU community on our clinical programs.” The HMI-103 pheEDIT trial is expected to enroll up to nine patients ages 18-55 years old who have been diagnosed with PKU due to PAH deficiency. Once positive safety and efficacy results are established in the adult population, Homology plans to enroll younger patients in clinical trials. The Phase 1 dose-escalation trial is designed to evaluate three doses of HMI-103 to determine the recommended dose(s) for a future trial. In addition to safety endpoints, the trial will measure serum phenylalanine (Phe) changes. HMI-103 is designed as a one-time administration to maximize the expression of functional phenylalanine hydroxylase (PAH) in liver cells and thus restore the natural biochemical pathway that metabolizes Phe. The product candidate was developed to specifically integrate a functional PAH gene into the genome using the natural DNA repair process of homologous recombination. In addition to expression, the integration is designed to correct the cell by inactivating at least one of the mutated genes and enable that correction to persist through cell division. Homology expects that the first patient in the pheEDIT clinical trial will be dosed following requisite Institutional Biosafety Committee and Institutional Review Board approvals at the clinical sites. The trial will include an 82-day screening/run-in period prior to HMI-103 administration. Homology also announced today an update from its ongoing Phase 2 pheNIX clinical trial evaluating HMI-102 gene therapy in adults with PKU. As of September 30, 2021, both doses in the trial have been generally well-tolerated and have shown evidence of biological activity, including clinically meaningful reductions in Phe levels, increases in Tyr and reductions in the Phe-to-Tyr ratio. Several new clinical trials sites have also recently been added to pheNIX for a total of 13 with more sites expected shortly. Despite increased interest, enrollment is slower than anticipated due in part to COVID-19 resurgence, and the Company anticipates providing a more detailed pheNIX update in mid-2022 when it expects to have a larger dataset. “We continue to be encouraged by the data from our pheNIX trial and to hasten enrollment, we have expanded our Medical Affairs, Clinical Development and Operations teams to support not only pheNIX, but now pheEDIT and our Hunter syndrome gene therapy trial expected this year,” said Arthur Tzianabos, Ph.D., President and Chief Executive Officer of Homology Medicines. “The flexibility of our technology platform enables us to develop gene therapy and gene editing product candidates as potential one-time treatments, and news today of our first trial that incorporates gene editing for PKU demonstrates our ability to employ both approaches in an effort to support the PKU community.” PKU is caused by mutations in the PAH gene, which is responsible for producing the enzyme that metabolizes Phe from dietary protein. As a result, Phe accumulates to toxic levels in the blood and brain and does not convert to melanin or the amino acid tyrosine (Tyr), a precursor to neurotransmitters. Homology’s approach to PKU with gene therapy and gene editing candidates is designed to treat both the adult and pediatric communities with one-time treatments that address the genetic cause of PKU. Since gene therapy does not integrate into the genome, it can be used in cells that are not rapidly dividing, such as an adult liver. Gene editing is designed to make a permanent correction in cells, including those that are rapidly dividing, such as a child’s liver. About HMI-102 Gene Therapy and HMI-103 Gene Editing Product Candidates HMI-102 is an investigational gene therapy in clinical development for the treatment of phenylketonuria (PKU) in adults. HMI-102 is designed to encode the PAH gene, which is mutated in people with PKU, and delivered via the liver-tropic AAVHSC15 vector. Homology has received Fast Track Designation and orphan drug designation for HMI-102 from the U.S. Food and Drug Administration (FDA), and orphan drug designation from the European Medicines Agency (EMA). HMI-103 is an investigational, nuclease-free gene editing product candidate ultimately designed to treat pediatric patients with PKU, whose livers are rapidly dividing, following initial clinical trials in adults. HMI-103 also uses AAVHSC15 and is designed to encode the PAH gene flanked by homology arms, or long stretches of DNA, to target the PAH region of the genome. Using the body’s natural DNA repair process of homologous recombination, the PAH gene integrates into the genome. HMI-103 is designed to express phenylalanine hydroxylase (PAH) and integrate into the PAH gene to restore the natural biochemical pathway that metabolizes phenylalanine (Phe). About Phenylketonuria (PKU)PKU is a rare inborn error of metabolism caused by a mutation in the PAH gene. PKU results in a loss of function of the enzyme phenylalanine hydroxylase (PAH), which is responsible for the metabolism of phenylalanine (Phe), an amino acid obtained exclusively from the diet. If left untreated, toxic levels of Phe can accumulate in the blood and result in progressive and severe neurological impairment. Currently, there are no treatment options for PKU that target the underlying genetic cause of the disease. According to the National PKU Alliance, PKU affects nearly 16,500 people in the U.S. with approximately 350 newborns diagnosed each year. The worldwide prevalence of PKU is estimated to be 50,000 people. About Homology Medicines, Inc. Homology Medicines, Inc. is a clinical-stage genetic medicines company dedicated to transforming the lives of patients suffering from rare diseases by targeting the underlying cause of the disease. The Company’s lead clinical program, HMI-102, is an investigational gene therapy for adults with phenylketonuria (PKU) and additional programs focus on gene editing in PKU, lysosomal storage disorders including Hunter syndrome, paroxysmal nocturnal hemoglobinuria (PNH) and other diseases. Homology’s proprietary platform is designed to utilize its family of 15 human hematopoietic stem cell-derived adeno-associated virus vectors (AAVHSCs) to precisely and efficiently deliver genetic medicines in vivo through a gene therapy or nuclease-free gene editing modality, as well as to deliver one-time gene therapy to produce antibodies throughout the body through the GTx-mAb platform. Homology has a management team with a successful track record of discovering, developing and commercializing therapeutics with a focus on rare diseases and believes that its data, internal manufacturing capabilities and broad intellectual property position the Company as a leader in genetic medicines. For more information, visit www.homologymedicines.com. Forward-Looking Statements This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including without limitation statements regarding our expectations surrounding the potential, safety, efficacy, and regulatory and clinical progress of our product candidates; the potential of our gene therapy and gene editing platforms, including our GTx-mAb platform; our expectations surrounding clinical trial enrollment; our plans and timing for the commencement of and the release of data from clinical trials; our beliefs regarding our manufacturing capabilities; and our position as a leader in the development of genetic medicines. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements, including, but not limited to, the following: the impact of the COVID-19 pandemic on our business and operations, including our preclinical studies and clinical trials, and on general economic conditions; we have and expect to continue to incur significant losses; our need for additional funding, which may not be available; failure to identify additional product candidates and develop or commercialize marketable products; the early stage of our development efforts; potential unforeseen events during clinical trials could cause delays or other adverse consequences; difficulties or delays enrolling patients in clinical trials; risks relating to the capabilities of our manufacturing facility; risks relating to the regulatory approval process; interim, topline and preliminary data may change as more patient data become available, and are subject to audit and verification procedures that could result in material changes in the final data; our product candidates may cause serious adverse side effects; inability to maintain our collaborations, or the failure of these collaborations; our reliance on third parties; failure to obtain U.S. or international marketing approval; ongoing regulatory obligations; effects of significant competition; unfavorable pricing regulations, third-party reimbursement practices or healthcare reform initiatives; product liability lawsuits; failure to attract, retain and motivate qualified personnel; the possibility of system failures or security breaches; risks relating to intellectual property and significant costs as a result of operating as a public company. These and other important factors discussed under the caption “Risk Factors” in our Quarterly Report on Form 10-Q for the quarter ended June 30, 2021 and our other filings with the SEC could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management’s estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, we disclaim any obligation to do so, even if subsequent events cause our views to change. Company ContactsTheresa McNeelyChief Communications Officer and Patient Advocatetmcneely@homologymedicines.com781-301-7277 Media Contact:Cara Mayfield Vice President, Patient Advocacy and Corporate Communications cmayfield@homologymedicines.com 781-691-3510