Association of an X-chromosome dodecamer insertional variant allele with mental retardation

Mental retardation is a prominent feature of many neurodevelopmental syndromes. In an attempt to identify genetic components of these illnesses, we isolated and sequenced a large number of human genomic cosmid inserts containing large trinucleotide repeats. One of these cosmids, Cos-4, maps to the X-chromosome and contains the sequence of a 7.3-kb mRNA. Initial polymorphism analysis across a region of repetitive DNA in this gene revealed a rare 12-bp exonic variation (1% in non-ill males) having an increased prevalence in non-Fragile X males with mental retardation (4%, P