Human KMT2C, MADD and TCF4 are causative genes for three neurodevelopmental disorders, Kleefstra syndrome 2, DEEAH syndrome and Pitt-Hopkins syndrome, respectively (Zweier et al., 2007; Kleefstra et al., 2012; Schneeberger et al., 2020). Here, MADD is linked to Kleefstra syndrome 2.