This study looked at cancers that have lost a gene called RB1, which normally acts as a brake on cell division. When this brake is missing, cancer cells become especially dependent on another protein called CDK2 to keep dividing. We found this dependency across many cancer types, but it is strongest when cells also have extra copies of a gene called CCNE1. The most important group that could benefit is patients with small cell lung cancer, where nearly all tumors have lost RB1 and there are currently no targeted treatments available. A new drug called INX-315 that blocks CDK2 is already in clinical trials and may help these patients.