Third Rock Ventures was founded by ex-Millennium Pharmaceuticals leaders to fill the gap left when Big Pharma and traditional VCs stopped funding very early-stage drug discovery, operating as a purpose-built biotech company creator that originates almost all of its own companies in-house rather than backing external business plans. The firm is known for going "all in" on a small number of ambitious bets in ultra-orphan and rare genetic disorders, often spending several years and millions of dollars incubating a single idea before launch and committing to very long timelines that can stretch beyond traditional VC fund cycles.