Zeta’s rabbit recombinant antibody recognizes Retinoblastoma (Rb), a well-characterized tumor suppressor gene product. The Retinoblastoma (Rb) antibody recognizes the 105kDa phosphoprotein and shows no cross reaction with p107 or p130. The Zeta recombinant antibody to Retinoblastoma (Rb) reacts with the hyper-phosphorylated as well as the under or unphosphorylated form of the Rb protein. The Retinoblastoma (Rb) gene product plays a key role in cell cycle control. Retinoblastoma has been identified as a tumor suppressor gene whose loss of its function leads to tumor development. Rb is widely expressed in a variety of human tissues including breast, esophageal, squamous cell and cervical carcinoma. Importatnly, some tumors that contain Retinoblastoma (Rb) gene mutations, are negative for Rb, including spindle cell lipoma and breast myofibroblastomas.