NKX3-2 and osteoarthritis: The HDAC9-PIASy-RNF4 axis promotes chondrocyte hypertrophy by regulating the sumo and ubiquitination of Nkx3.2/Bapx1, which is degraded by the proteasome, according to studies on HDAC9 in osteoarthritis (Choi et al., 2016).