These findings provide evidence that nicotinamide primarily targets SIRT1 for negative regulation in cancer cell survival, which eventually triggers 1) S phase arrest through the restoration of RB and p53–p21 (TP53-CDKN1A) axis based activity and G2/M phase disturbances through the FOXM1-cyclin B1 (CCNB1) axis, 2) the execution of apoptosis and 3) the widespread dysfunction of DNA damage repair. The gene discussed is CDKN1A; the disease is cancer.