Despite genetic evidence implicating MBD5 as the only overlapping gene between various 2q23.1 microdeletions, the function of MBD5 and its causality to 2q23.1 microdeletion syndrome, a disorder characterized by developmental delay and autistic features, has yet to be determined. In this issue of EMBO Molecular Medicine, Camarena et al generate an Mbd5 gene-trap mouse model and show for the first time that mice with reduced MBD5 expression develop behavioral abnormalities with neuronal function deficits, mimicking symptoms in 2q23.1 microdeletion syndrome, thus supporting a causal role for MBD5 haploinsufficiency in the disorder.