CCND1 and prostate cancer: Therefore, we hypothesized that polymorphisms of genes encoded cyclin-dependent kinase 4 and 6 (CDK4 and CDK6), cyclin D1 (CCND1), CDK inhibitors p16INK4a and p15INK4b, as well as the retinoblastoma protein (RB), could modulate prostate cancer risk and influence the corresponding mRNA levels.