Class I HDAC proteins are mainly distributed in the nucleus, among which HDAC1 and HDAC2 are highly expressed in lung cancer, breast cancer, gastric cancer, ovarian cancer, colorectal cancer, and pancreatic cancer.366–370 Class II HDAC proteins are mainly distributed in the cytoplasm and can shuttle from the cytoplasm to the nucleus, and HDAC4 and HDAC6 are highly expressed in liver cancer and oral squamous cell carcinoma, respectively.371,372 Class III contains members of the sirtuin family, which are usually responsible for the deacetylation of nonhistone proteins. Here, HDAC9 is linked to breast cancer.