In normal mammary tissue cells, in benign tumor cells, and in low malignancy grade carcinomas (G1 carcinomas), SIRT1 immunoreactivity had strong throughout and was preferentially nuclear; on the other hand, in less differentiated, highly malignant cancer cells (G2–G3 carcinomas), SIRT1 immunoreactivity was generally weaker but shifted to cytoplasmic localization, as shown on tissues in IHC. This evidence concerns the gene SIRT1 and cancer.