He is a physician-scientist who studies the nature and causes of developmental brain disorders, and is a leading authority on the causes of human brain malformations. While best known for his work on lissencephaly (smooth brain) and megalencephaly (enlarged brain), he provided the original descriptions for more than 10 clinically recognizable and now well-known syndromes including Smith-Magenis syndrome, rapid-onset dystonia-parkinsonism, coloboma-renal syndrome, bilateral frontoparietal cobblestone malformation, megalencephaly-capillary malformation syndrome (MCAP), megalencephaly-polymicrogyria-polydactly-hydrocephalus syndrome (MPPH) and X-linked lissencephaly with abnormal genitalia (XLAG), all of these now linked to the underlying gene or genes. He co-discovered at least 14 pathogenic copy number variants including the well-known deletion 17p13.3 (Miller-Dieker syndrome), deletion 17p11.2 (Smith-Magenis syndrome) and deletion 16p11.2 (autism and intellectual disability), and co-discovered almost 70 human disease genes.