Mutations in GRIN1 (which encodes the GluN1 subunit), GRIN2B (GluN2B), and GRIN2D (GluN2D), expressed during embryonic development, display more severe clinical phenotypes, including severe intellectual disability and developmental delay, than GRIN2A (GluN2A) mutations. The gene discussed is GRIN2D; the disease is Global developmental delay.