HDAC4 and 2q37 microdeletion syndrome: In humans, HDAC4 haploinsufficiency leads to brachydactyly mental retardation syndrome (BDMR, OMIM: 600430), also known as 2q37 deletion syndrome, characterized by craniofacial abnormalities, including a prominent forehead, midface retrusion, microcephaly, and a depressed nasal bridge (Williams et al., 2010).