Wiedemann-Steiner syndrome (WSS, MIM #605130) is a Mendelian Disorder of Epigenetic Machinery (MDEM) most often caused by haploinsufficiency of KMT2A, a gene encoding a histone methyltransferase that adds histone 3 lysine 4 (H3K4) methylation (Jones et al., 2012). The gene discussed is KMT2A; the disease is Wiedemann-Steiner syndrome.