GPC3 and Macrocephaly: Methods: We carried out a retrospective analysis of a cohort of 78 patients who were tested from February 2017 to December 2024 by high-throughput sequencing of a panel of 27 genes (ABCC9, AKT1, AKT2, AKT3, BRWD3, DIS3L2, DNMT3A, EZH2, GPC3, GPC4, HERC1, MED12, MTOR, NFIA, NFIX, NSD1, PDGFRB, PIK3CA, PIK3R1, PIK3R2, PPP2R1A, PPP2R5D, PTEN, RAB39B, RNF135, SETD2, and TBC1D7) because of neurodevelopmental impairment, including ID/DD, ASD/behaviour abnormalities associated with macrocephaly, mimicking to a large extent idiopathic ASD.