Kleefstra Syndrome is the result of mutations in the H3K9me3 writer EHMT1 (also called CLR4 or G9A) (Frega et al., 2019) and Rett Syndrome is caused by mutations in the MECP2 gene that codes for a DNA-methylase that modifies the DNA where it contacts H3K9 residues (Fuks et al., 2003; Thambirajah et al., 2012; Bian et al., 2019). Here, MECP2 is linked to Rett syndrome.