TWIST1 and microcephaly: Syndromes with MC may be sub-classified into seven main etiological groups including: common craniofacial syndromes related to known craniofacial genes (FGFR family, TWIST); other recognized clinical genetic syndromes resulting in microcephaly/microencephaly; developmental disorders of the midline; recurrent chromosomal/genomic copy number imbalances; syndromes with abnormal metabolic function; teratogenic/environmental causes; and an idiopathic group (Table S1).