Uncommon Therapeutics

Uncommon Therapeutics is a San Francisco-based biotechnology company founded in 2024 that develops drugs targeting the underlying causes of rare ("uncommon") genetic diseases. Its first indication is Rett Syndrome, a monogenetic neurodevelopmental disease that the company states affects 1 in 10,000 girls. Rather than building a broad discovery platform, the company organizes itself around a single disease, aiming to develop several distinct high-value therapeutics for it that, used together as a polytherapy, could constitute a cure.

The company's stated approach is to combine deep domain expertise in the disease with modality validation in disease models it believes will translate to patients, then advance candidates to the clinic with focused capital. Work spans gene editing, antisense oligonucleotide (ASO) and small-molecule approaches, alongside a repurposed-drug polytherapy strategy that the founders have applied outside a formal trial setting. Uncommon participated in Y Combinator's Winter 2025 batch, where it is categorized under gene therapy, CRISPR and biotechnology; its YC primary partner is Jared Friedman. As of that listing the team consisted of two people.

Founding story

Co-founder and CEO Noah Auerhahn, previously a technology entrepreneur, turned to rare disease after his 18-month-old daughter Ellie was diagnosed with Rett Syndrome in 2021 and began losing the ability to walk, speak and use her hands. He read more than 300 papers in the first nine months and consulted disease experts, then identified several repurposed drugs not commonly used in Rett that improved her quality of life; the company reports she is now 5 years old, without seizures, walking, and learning to read and communicate with an eye-gaze device. Auerhahn personally invested over $1M into these efforts before the company was founded in 2024 with co-founder and CSO Ryan Lim, PhD.

Business model

Asset-focused biotechnology: the company develops its own drug candidates for a single disease rather than licensing a discovery platform, relying on external contract research organizations for stages of drug development and on consultants with platform-company experience. It states it seeks partnerships with biotech executives working on rare disease.

Traction

The company reports that with approximately $1M it designed three novel therapeutics targeting the underlying cause of Rett Syndrome, all of which are working in patient cells. Separately, a repurposed-drug polytherapy identified by the founder is described as having improved the quality of life of the CEO's daughter. Team size was reported as two.

Latest developments

Uncommon Therapeutics emerged from stealth ahead of Y Combinator's Winter 2025 Demo Day on 12 March 2025 and published a launch post describing its Rett Syndrome program and three designed therapeutics active in patient cells.