Chromosomal protein that binds to methylated DNA. It can bind specifically to a single methyl-CpG pair. It is not influenced by sequences flanking the methyl-CpGs. Mediates transcriptional repression through interaction with histone deacetylase and the corepressor SIN3. Binds both 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC)-containing DNA, with a preference for 5-methylcytosine (5mC). Interacts with FNBP3 (PubMed:9171351). Interacts with CDKL5 (By similarity). Interacts with ATRX; MECP2 recruits ATRX to pericentric heterochromatin in neuronal cells (PubMed:17296936). Interacts with NCOR2 (PubMed:23770565). Interacts with TBL1XR1; bridges interaction between MECP2 and NCOR1 (PubMed:28348241). Interacts with TBL1X; recruits TBL1X to the heterochromatin foci (By similarity). Colocalized with methyl-CpG in the genome. Colocalized with TBL1X to the heterochromatin foci. Phosphorylated on Ser-421 by CaMK2 in brain upon synaptic activity, which attenuates its repressor activity and seems to regulate dendritic growth and spine maturation. Does not seem to be phosphorylated on Ser-421 in other tissues. Ten times higher expression levels than isoform A in brain (at protein level).