The tumour suppressor protein p21 can regulate the phosphorylation of retinoblastoma and subsequently block of DNA replication to prevent endoreduplication in different cancer cells at both the G1/S and the G2/M cell cycle transitions.32 Furthermore, p21, p27Kip1 and cyclin D1 play important roles in regulating the cell cycles. The gene discussed is CCND1; the disease is cancer.