G1/S-specific cyclin-D1 (cyclin D1) is a regulatory component of the cyclin D1-CDK4 (DC) comple, Xenopus/Amphibian, that phosphorylates and inhibits members of the retinoblastoma (RB) protein family including RB1 and regulates the cell-cycle during G1/S transition. Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F comple, Xenopus/Amphibian, and the subsequent transcription of E2F target genes which are responsible for the progression through the G1 phase. Defects in cyclin D1 are a cause of multiple myeloma (MM). MM is a malignant tumor of plasma cells usually arising in the bone marrow and characterized by diffuse involvement of the skeletal system, hyperglobulinemia, Bence-Jones proteinuria and anemia.