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The retinoblastoma gene family consists of the tumor suppressor protein pRB and its two relatives p107 and p130. These proteins have been implicated in the regulation of cell cycle progression, in part, through inactivation of members of the E2F transcription factor family. Overexpression of pRB, p107, or p130 leads to growth arrest in the G₁ phase of the cell cycle, and this arrest is abolished by complex formation with the adenovirus E1A, human papilloma virus E7, or simian virus 40 T oncoproteins. Inactivation of pRB by gross structural alterations or point mutations in the RB-1 gene has been described in a variety of human tumors, including retinoblastomas, osteosarcomas, and small cell lung carcinomas. Despite the structural and functional similarity between pRB, p107, and p130, alterations in the latter two proteins have not been identified in human tumors. We have screened a panel of 17 small cell lung …