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Abstract

Most patients with mental retardation (MR) are nonsyndromic; i.e. they either have no accompanying clinical, radiological, or metabolic abnormalities, or their additional features are not specific enough to enable allocation to a recognizable malformation syndrome. Numerous novel disease genes for X-chromosomal nonsyndromic MR (NS-MR) have been elucidated in recent years, and research into autosomal forms of NS-MR has yielded first results. Both forms have turned out to be characterized by extreme genetic heterogeneity. Routine diagnostic mutation screening in the known NS-MR genes is currently not feasible in sporadic patients but will be facilitated by novel sequencing technologies in the near future. Patients with familial NS-MR should be offered inclusion in ongoing research programmes. Several X-chromosomal NS-MR genes demand consideration in the routine diagnostic workup of MR patients because they overlap phenotypically with syndromic forms of MR.