The GRIN2B (glutamate receptor, ionotropic, N-methyl-d-aspartate 2B) gene, located in the short arm of chromosome 12, encoding the NR2B subunit of the N-methyl-D-aspartate receptor, has recently been recognized to play an important role in corticogenesis and brain plasticity. Deletions in the short arm of chromosome 12 are rare. Hemizygous loss of function of the GRIN2B gene results in developmental delay, whereas gain of function leads to epilepsy, and infantile spasms in particular. In addition, GRIN2B variants have been associated with autism spectrum disorder and schizophrenia. Here the authors report a child with global developmental delay, autistic behavioural features, central hypotonia, dysmorphic features and isolated congenital anomalies of the fingers and toes, and a de novo heterozygous deletion in chromosome locus 12p13.2-p13.1, involving loss of several genes, including GRIN2B. This report and our review of the literature help clarify the distinct phenotypes associated with loss or gain of GRIN2B function.