CDC25A and retinoblastoma: To determine the possible mechanisms of how PAX6 regulates cell cycle progression, we examined the mRNA and protein levels of several cell cycle regulatory genes, such as cdc25A, CDK2, PCNA CDK1 and p21. In the PAX6-knockdown cells, CDK2 expression was increased in the SO-Rb50 retinoblastoma cells, suggesting an induction into the S phase.