CHEK2 and retinoblastoma: We found a significant degree of upregulation of cell cycle checkpoint genes such as CDK1, CCNB2, CCNE2, CDKN2A, CDKN3, PTTG1, CHEK2, and RPA4 (p ≤ 0.05) in advanced Rb tumors compared to non-advanced Rb tumors (Figure 3A).