The present invention relates to newly discovered human histone deacetylases (HDACs), also referred to as histone deacetylase-like polypeptides. The polynucleotide sequences and encoded polypeptides of the novel HDACs are encompassed by the invention, as well as vectors comprising these polynucleotides and host cells comprising these vectors. The invention also relates to antibodies that bind to the disclosed HDAC polypeptides, and methods employing these antibodies. Also related are methods of screening for modulators, such as inhibitors or antagonists, or agonists. The invention also relates to diagnostic and therapeutic applications which employ the disclosed HDAC polynucleotides, polypeptides, and antibodies, and HDAC modulators. Such applications can be used with diseases and disorders associated with abnormal cell growth or proliferation, cell differentiation, and cell survival, e.g., neoplastic cell growth, and especially breast and prostate cancers or tumors.