FDA creates new approval pathway for gene therapies tailored to individual rare disease patients

Overview

For decades, the Food and Drug Administration (FDA) required the same basic proof for every drug: show it works in a controlled trial with enough patients to be statistically meaningful. That standard made sense for common diseases but created an impossible barrier for conditions affecting a handful of people worldwide. On February 23, 2026, the FDA issued draft guidance creating a fundamentally different standard—called the "plausible mechanism" framework—that would let developers of individualized gene-editing and ribonucleic acid (RNA) therapies win full approval by demonstrating their treatment targets the root genetic cause, successfully edits or engages the target, and improves outcomes compared to the disease's documented natural course.

The framework matters because the science of bespoke genetic medicine has outrun the regulatory system built to evaluate it. Researchers have already shown they can identify a patient's unique mutation, design a custom therapy, and deliver it within months—as they did for an infant known as Baby KJ, who received the first personalized CRISPR gene-editing therapy in early 2025. But without a formal approval pathway, each such treatment exists in regulatory limbo. The new guidance, open for 60 days of public comment, would eliminate the requirement that diseases with hundreds of different mutations each run separate clinical trials, instead allowing a single framework to cover therapies sharing the same mechanism across different individual mutations.

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Key Indicators

7,000+

Known rare diseases

Roughly 7,000 rare diseases have been identified, approximately 80% of which are genetic in origin

300M

People affected globally

An estimated 300 million people worldwide live with a rare disease

~5%

Diseases with approved treatments

Fewer than 5% of known rare diseases have an FDA-approved therapy

60 days

Public comment period

The draft guidance is open for public comment before potential finalization

Voices

Curated perspectives — historical figures and your fellow readers.

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People Involved

Marty Makary

In office; co-architect of the plausible mechanism pathway

Vinay Prasad

Returned to CBER director role after brief departure in mid-2025

Tracy Beth Høeg

Leading CDER's role in the new framework

Timothy Yu

Pioneer of individualized antisense oligonucleotide therapies

Organizations Involved

U.S. Food and Drug Administration

Federal regulator

Issued draft guidance creating the plausible mechanism framework

The United States agency responsible for approving drugs, biologics, and medical devices for public use.

N=1 Collaborative

Research Consortium

Coordinating platform for individualized therapy development

An international network of researchers, clinicians, and patient advocates working to build scalable pathways for individualized genetic medicines.

Children's Hospital of Philadelphia

Pediatric Academic Medical Center

Site of the first personalized CRISPR therapy for Baby KJ

The pediatric hospital where researchers designed and delivered the first individualized CRISPR base-editing therapy, treating Baby KJ for a fatal urea cycle disorder in early 2025.

Timeline

January 1983 February 2026

8 events Latest: February 23rd, 2026 · 3 months ago

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February 2026
FDA issues draft guidance for individualized therapy framework
Latest Regulatory

The FDA released the formal draft guidance creating the plausible mechanism pathway, allowing developers of individualized gene-editing and RNA therapies for ultra-rare diseases to seek full approval using mechanistic rationale and natural history comparisons rather than traditional clinical trials.
February 23rd, 2026
November 2025
Makary and Prasad publish plausible mechanism concept in NEJM
Publication

FDA Commissioner Makary and CBER Director Prasad outlined the plausible mechanism pathway in the New England Journal of Medicine, citing Baby KJ as the model for the kind of individualized therapies the framework would enable.
November 12th, 2025
September 2025
FDA introduces Rare Disease Evidence Principles
Regulatory

The FDA established new evidence standards for ultra-rare diseases affecting fewer than 1,000 Americans, allowing approval based on a single pivotal trial combined with mechanistic or biomarker evidence.
September 3rd, 2025
February 2025
Baby KJ receives first personalized CRISPR therapy
Medical Milestone

An infant with CPS1 deficiency—a fatal urea cycle disorder—received a custom CRISPR base-editing therapy at Children's Hospital of Philadelphia, becoming the first patient treated with a bespoke gene-editing drug.
February 25th, 2025
July 2024
FDA announces Rare Disease Innovation Hub
Regulatory

The FDA created a cross-agency hub to coordinate expertise between its drug and biologics centers, with particular focus on treatments for small patient populations where natural history data is limited.
July 1st, 2024
October 2019
Milasen results published in the New England Journal of Medicine
Publication

Yu's team published the first peer-reviewed account of a patient-customized antisense therapy, establishing a precedent that would inform future n-of-1 treatment protocols.
October 9th, 2019
January 2018
Milasen: first individualized antisense drug administered
Medical Milestone

Timothy Yu's team at Boston Children's Hospital began treating Mila Makovec, a six-year-old with fatal Batten disease, with a custom antisense oligonucleotide designed in under a year. The case proved individualized genetic therapy was feasible.
January 1st, 2018
January 1983
Orphan Drug Act becomes law
Legislation

President Reagan signed the Orphan Drug Act, offering tax credits, market exclusivity, and grants to incentivize development of drugs for diseases affecting fewer than 200,000 Americans. Before the law, only 38 drugs existed for rare diseases.
January 4th, 1983

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

January 1983

Orphan Drug Act (1983)

Before 1983, pharmaceutical companies had little financial incentive to develop drugs for diseases affecting small populations—only 38 such drugs existed in the United States. Congressman Henry Waxman shepherded the Orphan Drug Act through Congress, and President Reagan signed it on January 4, 1983, creating tax credits, seven-year market exclusivity, and development grants for drugs targeting diseases affecting fewer than 200,000 Americans.

Then

Drug development for rare diseases began immediately. Within a decade, the FDA had granted hundreds of orphan drug designations.

Now

Over four decades, the FDA granted more than 6,300 orphan drug designations and approved treatments covering nearly 400 rare diseases. Japan and the European Union adopted similar laws in 1993 and 2000.

Why this matters now

The Orphan Drug Act proved that regulatory incentives could redirect pharmaceutical investment toward neglected diseases. The plausible mechanism framework tackles the next barrier the 1983 law could not solve: what happens when the patient population is too small for even a modified clinical trial.

January 2017 – October 2019

Milasen and the birth of n-of-1 genetic therapy (2018)

Neurologist Timothy Yu at Boston Children's Hospital saw a Facebook post about Mila Makovec, a six-year-old girl in Colorado dying of Batten disease caused by a unique genetic mutation. In ten months, his team identified the mutation, designed a custom antisense oligonucleotide to bypass it, manufactured the drug, obtained FDA authorization, and began treatment—all for a single patient. The drug, named milasen, reduced Mila's seizures.

Then

Mila's seizures decreased. The case, published in the New England Journal of Medicine in October 2019, demonstrated that individualized genetic therapy could move from diagnosis to treatment in under a year.

Now

Mila died in February 2021 at age ten, but her case opened the door for more than 85 additional patients to receive individualized antisense therapies and led to creation of the N=1 Collaborative, a network coordinating the field.

Why this matters now

Milasen proved the scientific concept but exposed the regulatory gap: each individualized treatment required ad hoc FDA authorization because no formal approval pathway existed. The plausible mechanism framework directly addresses this gap.

1992

Accelerated approval pathway established (1992)

Amid the AIDS crisis, the FDA created the accelerated approval pathway allowing drugs to reach market based on surrogate endpoints—laboratory markers reasonably likely to predict clinical benefit—rather than waiting for definitive outcome data. The pathway was formalized under regulations that required post-approval confirmatory trials.

Then

Multiple HIV/AIDS drugs reached patients years earlier than traditional approval would have allowed, materially changing survival rates.

Now

Accelerated approval became a standard tool for cancer drugs, rare diseases, and other conditions with unmet need. The pathway has been used for more than 300 approvals, though it also generated debate about confirmatory trial requirements.

Why this matters now

Accelerated approval showed the FDA could maintain safety standards while accepting different forms of evidence when patient need demanded it. The plausible mechanism pathway extends this logic further, accepting mechanistic rationale as primary evidence when even surrogate-endpoint trials are impossible due to tiny patient populations.