a | Ribs from neonatal mice stained for bone (red) and cartilage (blue). Deletion of HDAC4 results in ossification of cartilage by the arrowhead), whereas overexpression of HDAC4 in the cartilage of transgenic mice prevents ossification. b | Schematic of the repressive influence of HDAC4 on myocyte enhancer factor 2 (MEF2) and runt related transcription factor 2 (RUNX2) in the pathway for chondrocyte proliferation and hypertrophy. IHH, Indian hedgehog; KO, knockout; PTHrP, parathyroid hormone-related peptide. Part a is reproduced, with permission, from REF. 33 © (2007) Cell Press.