CDKN1A and glioma: Because our data suggested that PER1 positively influenced the programmed cell death of U343 glioma cells, we subsequently examined the expression of the following genes known to be important in DNA damage repair and programmed cell death: ATM, CHK2, p21, P53, cyclinB1, cdc2, and C-MYC. As shown in Figure 4, shRNA treatment resulted in a clear reduction in the expression of the per1 protein compared to the control treatment.