CDK1 and neoplasm: At the same time, we examined the expression of PCNA protein as well as that of the cell cycle-related proteins cyclin A, cyclin E, CDK1, and CDK2 in the tumor tissues of circMAST1 siRNA- and control siRNA-injected mice; all were significantly lower in the circMAST1 siRNA group than the control siRNA group (Fig. 5d).