SIRT1 and breast cancer: Since previous studies indicated that the heterozygous deletion of another sirtuin family member, Sirt1, increases mammary tumor latency by interfering with the normal development of the mammary gland [32], we monitored the mammary gland development in MMTV-PyMT+/−; Sirt6+/+ and MMTV-PyMT+/−; Sirt6+/− mice.