Mutations leading to the loss of the retinoblastoma transcriptional corepressor 1 gene (RB1) or deregulation of its gene product, the retinoblastoma tumor suppressor protein (pRB), are associated with several forms of cancer including retinoblastoma, osteosarcoma, adenocarcinomas, small cell lung cancer, breast cancer, prostate cancer, and others [1]. The gene discussed is RB1; the disease is breast cancer.