In both cases a deletion affecting the same genes: the human homologue of the Drosophila flightless-I gene (FLII); cytosolic serine hydroxymethyltransferase (SHMT1); 21,23 the human homologue of Drosophila lethal 2 giant larva (LLGL1); and 18 topoisomerase IIIa (TOP3A), was found confirming the clinical diagnosis of SMS. The gene discussed is TOP3A; the disease is Smith-Magenis syndrome.