5. For HDACs 4, 5 and 7, conserved binding domains have been discovered that bind for C-terminal binding protein (CtBP), myocyte enhancer factor 2 (MEF2) and 14-3-3. All three HDACs work to repress the myogenic transcription factor MEF2 which an essential role in muscle differentiation as a DNA binding transcription factor. Binding of HDACs to MEF2 inhibits muscle differentiation, which can be reversed by action of Ca2+/calmodulin-dependent kinase (CaMK) which works to dissociate the HDAC/MEF2 complex by phosphorylating the HDAC portion. They have been seen to be involved in cellular hypertrophy in muscle control differentiation as well as cellular hypertrophy in muscle and cartilage tissues. HDACs 5 and 7 have been shown to work in opposition to HDAC4 during muscle differentiation regulation so as to keep a proper level of expression. There has been evidence that these HDACs also interact with HDAC3 as a co-recruitment factor to the SMRT/N-CoR factors in the nucleus. Absence of the HDAC3 enzyme has shown to lead to inactivity which makes researchers believe that HDACs 4, 5 and 7 help the incorporation of DNA-binding recruiters for the HDAC3-containing HDAC complexes located in the nucleus. When HDAC4 is knocked out in mice, they suffer from a pronounced chondrocyte hypertrophy and die due to extreme ossification. HDAC7 has been shown to suppress Nur77-dependent apoptosis. This interaction leads to a role in clonal expansion of T cells. HDAC9 KO mice are shown to suffer from cardiac hypertrophy which is exacerbated in mice that are double KO for HDACs 9 and 5. Class IIB The Class IIB HDACs include HDAC6 and HDAC10. These two HDACs are most closely related to each other in overall sequence. However, HDAC6's catalytic domain is most similar to HDAC9. A unique feature of HDAC6 is that it contains two catalytic domains in tandem of one another. Another unique feature of HDAC6 is the HDAC6-, SP3, and Brap2-related zinc finger motif (HUB) domain in the C-terminus which shows some functions related to ubiquitination, meaning this HDAC is prone to degradation. HDAC10 has two catalytic domains as well. One active domain is located in the N-terminus and a putative catalytic domain is located in the C-terminus along with an NES domain. Two putative Rb-binding domains have also been found on HDAC10 which shows it may have roles in the regulation of the cell cycle. Two variants of HDAC10 have been found, both having slight differences in