Individuals carrying biallelic pathogenic variants in these genes (ie, MED17, MED20, MED27) share similar clinical phenotypes, variably characterized by global developmental delay, seizures (often refractory), congenital or postnatal microcephaly, recurrent hearing and/or vision abnormalities, and CNS atrophy/degeneration, particularly affecting the cerebellum and the brainstem. This evidence concerns the gene MED20 and microcephaly.