The genetic basis of the relatively mild myopathic symptoms exhibited in a male was investigated. Mutation screening of a candidate gene, MTM1, represented a chance of establishing the molecular defect and the mode of inheritance. SSCA detected variation of the exon b PCR products from the proband and his mother, compared to that observed upon analysis of the PCR products from other members of the family and 159 unrelated X chromosomes. Sequencing revealed a C775 to T transition, in the proband and his mother, but not in his unaffected brother. To confirm the presence of a base change in this region, a Cfol site was introduced into the PCR product of the wildtype allele by using the forward primer 5'-AGAAAATAAGACGGTCATTGcG-3' (mismatch base in small font) with the exon b reverse primer as used by Laporte et al (1996). Analysis of DNA from other members of the family using this method revealed that this is a new mutation in the proband's mother. This mutation would result in a Arg259->Cys substitution.