Loss-of-function mutations in SYNGAP1 resulting in SynGAP haploinsufficiency contribute to a constellation of symptoms of ID, DD, and seizures, which have recently been termed a neurodevelopmental disorder—MRD5 (Agarwal et al., 2019; Kilinc et al., 2022). Here, SYNGAP1 is linked to intellectual disability, autosomal dominant 5.