The FDA has issued several new guidance documents that will significantly affect drug development for rare diseases.[6][1] Among them is the February 23, 2026 "Plausible Mechanism Framework" guideline proposal, designed to accelerate the development and approval of individualized therapies for ultra-rare diseases with small patient populations where standard randomized clinical trials are not feasible.[1][3][5] These therapies include genome editing (eg, CRISPR), RNA-based treatments such as antisense oligonucleotides, and others that target a specific genetic, cellular, or molecular abnormality as the cause of the disease.[1][3][4] Key requirements include identification of an abnormality, demonstration of an intervention in a biological pathway, data on the natural course of the disease in untreated patients, confirmation of a successful intervention, and evidence of clinical benefit or improvement in biomarkers.[3][5] The documents provide clarity on acceptable study designs, including the Rare Disease Evidence Pathway (RDEP) and guidelines for cell and gene therapies in small populations.[6] Rare disease developers welcomed this clarity, which is intended to encourage innovation and access to treatment for patients with diseases affecting fewer than 1,000 people in the US.[6] The guidance is available for public comment within 60 days of publication in the Federal Register.[3]