Tuberous sclerosis complex (TSC) is a genetic disorder with a birth incidence of 1 in 6000, which is quite common for a ‘rare’ genetic disease. The two genes, their proteins and intracellular signalling pathways have been identified. TSC leads to multi-system involvement including benign tumours and very high rates of TSC-associated neuropsychiatric disorders. Some people with TSC have severe to profound intellectual disability, while others are intellectually very able. This paper reports correlations between particular kinds of mutations and the level of intellectual ability, opening the way for detailed biochemical testing of the proposed effects of TSC mutations on intellectual development and intellectual developmental disability. (By Prof Petrus J de Vries, http://jmg.bmj.com/content/52/12/815 )