Autism spectrum disorders (ASDs) are neurodevelopmental disorders that frequently arise from de novo genetic mutations. One of the genes whose mutation is known to correlate with ASD is GRIN2B, which encodes the GluN2B subunit of NMDA receptors. These glutamate receptors are required for neuronal development and plasticity; however, the mechanisms that lead to ASD pathophysiology upon GRIN2B mutation have so far remained elusive. Shasta Sabo and co-workers (Sceniak et al., 2019) now investigate this question using a mutant of GRIN2B that gives rise to a truncated GluN2B724t protein. The authors show that the GluN2B724t mutant does not form functional receptors, although it retains its interaction with the obligatory NMDA receptor subunit GluN1. Instead, GluN2B724t is missorted such that it localises largely to the soma of cortical neurons, whereas wild-type GluN2B forms puncta in the dendritic arbour and on the cell surface of neurons. Furthermore, the dendrites of cells containing the GluN2B724t mutant appeared highly dysmorphic, suggesting that GluN2B724t interferes with dendrite development. However, the mTOR pathway, which promotes dendrite growth and has been implicated in ASDs, is unaffected by the presence of GluN2B724t. Collectively, this work suggests that ASD pathophysiology in patients with a GRIN2B mutation results from the aberrant development of neuronal dendrites.