TP53 and cancer: The loss or mutation of p53 in many cancers leads to impaired cell cycle regulation, genomic instability, and the inhibition of apoptosis The protein levels of p53 in the per1-silenced cells tended to be lower than those in the control cells, indicating that per1 regulated p53 expression, and this activity may at least partially contribute to the apoptosis of per1-silenced cells.