Members of the retinoblastoma (Rb) family, including the related proteins p107 and p130, share several properties, including the ability to regulate E2F dependent transcription and to regulate cell-cycle progression. The Rb gene product is a phosphoprotein that is expressed in most normal cells of vertebrates. Rb acts as a tumor suppressor by providing a cell cycle checkpoint between the G1 and S phases. The tumor suppressor gene RB1, encoding the retinoblastoma (Rb) protein, is located at 13q14. By immunohistochemistry, Rb1 expression is defect/negative in soft tissue tumors associated with 13q deletions, all spindle cell lipomas, pleomorphic lipomas, and cellular angiofibromas and most mammary-type myofibroblastomas. Rb1 expression is intact in histologic mimics.