FDA Issues Draft Guidance on Using Next-Generation Sequencing to Assess Genome-Editing Safety

The Food and Drug Administration on Tuesday issued a draft guidance that spells out how drug developers should use next-generation sequencing to search for off-target genetic edits and chromosomal damage before testing genome-editing therapies in people, a move that quietly sets a technical benchmark for the field as CRISPR and similar tools move into mainstream medicine.

The document, titled “Safety Assessment of Genome Editing in Human Gene Therapy Products Using Next-Generation Sequencing,” lays out detailed expectations for how sponsors should design, run and report sequencing-based safety studies in support of investigational new drug applications and eventual license applications.

“Genome editing holds extraordinary promise for treating previously incurable genetic diseases,” FDA Commissioner Marty Makary said in a statement announcing the draft. The guidance reflects the agency’s view that realizing that promise will depend as much on rigorous safety infrastructure as on scientific breakthroughs.

Prepared by the Center for Biologics Evaluation and Research’s Office of Therapeutic Products, the April 14 draft is aimed at nonclinical, or preclinical, work that companies and academic centers submit to the agency before human trials. It applies broadly to genome-editing approaches used in somatic, or nonreproductive, cells, including both nuclease-based tools such as CRISPR and nuclease-independent editors that target DNA, RNA, or epigenetic marks.

The guidance “provides recommendations for next-generation sequencing (NGS)-based methods used in nonclinical studies that will likely be needed to support initiation of clinical trials of investigational human genome editing (GE) products,” the document states.

In practical terms, the FDA describes what it expects to see when sponsors use short- and long-read sequencing technologies to probe two central safety issues: unintended “off-target” edits elsewhere in the genome and disruptions to chromosomal integrity, such as rearrangements or large deletions. The recommendations cover ex vivo products, where cells are edited outside the body and then returned to patients, and in vivo approaches, which deliver editing tools directly into the body using viral vectors, lipid nanoparticles or other systems.

“Next-generation sequencing not only detects off-target editing and assesses chromosomal integrity;” it also underpins how sponsors characterize the risks of new gene-editing products, said Vinay Prasad, director of CBER, in the agency’s announcement.

The new draft builds on a January 2024 FDA guidance, “Human Gene Therapy Products Incorporating Human Genome Editing,” which set high-level expectations for product design, manufacturing, nonclinical safety packages and clinical trial planning. The latest document adds a methods-level layer to that framework, focusing specifically on how NGS should be deployed and what data regulators expect to see.

FDA staff outline expectations around sequencing strategies, sample selection, read length, depth, bioinformatics pipelines, reference databases and reporting formats. While the agency does not mandate particular platforms or numerical thresholds, observers say this level of specificity can effectively standardize the way sponsors measure and report off-target and chromosomal events — and, in turn, shape how preclinical programs are designed, budgeted and scheduled.

The guidance is formally nonbinding, part of the agency’s standard system of documents that describe its “current thinking” rather than enforceable rules. But companies typically treat such guidance as de facto requirements. If sponsors do not address the recommended NGS analyses, FDA reviewers can raise questions, request additional experiments or extend review timelines for investigational and license applications, potentially delaying clinical trials or approvals.

More extensive sequencing and analysis will likely increase upfront costs and technical complexity, particularly for smaller biotechnology companies and academic spinouts operating on limited capital. At the same time, developers and investors often seek clear regulatory expectations to reduce uncertainty, and some may view detailed NGS guidance as a way to de-risk long and expensive gene-editing programs.

The recommendations also have implications beyond drug sponsors. Clinical and research laboratories that run sequencing assays, as well as vendors that provide NGS platforms and bioinformatics services, could see demand shift toward approaches that align closely with FDA’s preferred study designs and control strategies.

The timing underscores how the questions around genome editing are changing. With therapies such as CASGEVY (exagamglogene autotemcel) already approved in the United States for sickle cell disease, many scientific debates have moved from whether gene editing can work to how to detect rare but potentially serious off-target events before and after patients are treated.

The guidance is explicitly limited to somatic editing but emphasizes the need to assess unintended edits, chromosomal stability and the risk of inadvertent effects on germ cells, which could theoretically be passed to future generations. Its cautious stance echoes the legacy of earlier setbacks in gene therapy, including the 1999 death of volunteer Jesse Gelsinger from an immune reaction to an experimental treatment and leukemia cases in early trials for X-linked severe combined immunodeficiency linked to retroviral vectors. Those incidents helped drive a more conservative regulatory posture that still shapes policy today.

The NGS document also fits into a broader FDA push to make individualized and ultra-rare disease therapies more feasible. On Feb. 23, the agency launched a “Plausible Mechanism Framework” intended to speed development for highly tailored treatments by clarifying what evidence is needed to justify trying them in patients. The April 14 draft is described as supporting that effort by providing a more robust template for safety assessment of the kind of bespoke genome-editing products those programs may generate.

Like other draft guidances, the NGS document is subject to public comment. Stakeholders have 90 days from its publication in the Federal Register to submit feedback through Regulations.gov, referencing docket number FDA-2026-D-1255, or by mail to the agency’s Dockets Management office. Contact information for CBER’s Office of Communication, Outreach and Development is included for technical and regulatory questions.

As gene-editing technologies proliferate, the agency’s move signals that the path from lab to patient will increasingly run through not just innovative science, but also standardized, data-heavy safety testing — with next-generation sequencing at its core.