Histone deacetylase (HDAC) and histone acetyltransferase (HAT) are enzymes thatregulate transcription by selectively deacetylating or acetylating the eta-amino groups oflysines located near the amino termini of core histone proteins (1). Eight members ofHDAC family have been identified in the past several years (2,3). These HDAC familymembers are divided into two classes, I and II. Class I of the HDAC family comprisesfour members, HDAC-1, 2, 3, and 8, each of which contains a deacetylase domainexhibiting from 45 to 93% identity in amino acid sequence. Class II of the HDAC familycomprises HDAC-4, 5, 6, and 7, the molecular weights of which are all about twofoldlarger than those of the class I members, and the deacetylase domains are presentwithin the C-terminal regions, except that HDAC-6 contains two copies of the domain,one within each of the N-terminal and C-terminal regions. Human HDAC-1, 2 and 3were expressed in various tissues, but the others (HDAC-4, 5, 6, and 7) showedtissue-specific expression patterns (3). These results suggested that each member ofthe HDAC family exhibits a different, individual substrate specificity and function in vivo. Alternate Names: anti-HD3 antibody, anti-RPD3 antibody, anti-RPD3-2 antibody.