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 SIN3A. Binds both 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC)-containing DNA, with a preference for 5-methylcytosine (5mC). Interacts with FNBP3. Interacts with CDKL5. Interacts with ATRX; MECP2 recruits ATRX to pericentric heterochromatin in neuronal cells. Interacts with NCOR2. Interacts with TBL1XR1; bridges interaction between MECP2 and NCOR1. 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-423 in brain upon synaptic activity, which attenuates its repressor activity and seems to regulate dendritic growth and spine maturation.