The mechanistic connection between PTHrP and HDAC4 also help explain why HDAC4 haploinsufficiency causes Brachydactyly mental retardation syndrome (21), which is phenotypically similar to Brachydactyly type E caused by mutations in PTHLH, the gene that encodes PTHrP (22). This evidence concerns the gene HDAC4 and brachydactyly type E.