In 2015, a whole exome sequencing (WES) analysis of a cohort of 46 individuals with an initial diagnosis of Coffin–Siris syndromes (CSS1, OMIM #135900; CSS2, OMIM #614607; CSS3, OMIM #614608; CSS4, OMIM #614609; CSS5, OMIM #616938; CSS6, OMIM #617808; CSS7, OMIM #618027; CSS8, OMIM #618362; CSS9, OMIM #615866; CSS10, OMIM #618506; CSS11, OMIM #618779; CSS12, OMIM #619325) or Nicolaides–Baraitser syndromes (NCBRS, OMIM #601358) revealed a heterozygous de novo missense mutation in the KMT2A gene in a boy clinically diagnosed with CSS1 [29]. Here, KMT2A is linked to Coffin-Siris syndrome 1.