Gene localization was determined by linkage analysis in 5 families with non-specific X-linked mental retardation (MRX) and were MRX1, Xp11.4-q21.31; MRX10, Xp21.3-p11.4; MRX11, Xp21.3-p11.22; MRX12, Xp21.3-q21.1; and MRX13, Xp22.3-q21.22. Four of these localizations cross the dystrophin brain promoter, a candidate locus for MRX. None of the affected individuals who were tested showed variation suggestive of a deletion. No consistent clinical features were observed between or within 4 of the 5 families. In MRX12, prematurity or low birth weight, hypotelorism and short stature were seen in several affected males. Heterozygote manifestations occurred in 3 families. There was no evidence to suggest involvement of the same gene in more than one family, nor to clinically separate these families into distinct genetic entities. Non-overlapping localizations for MRX1 and MRX10 demonstrate the existence of at least 2 separate loci among these 5 families.