The purpose of the present investigation was to assess the possibility of building a model to estimate, through dystrophin western blotting analysis, the expression of the DMD/BMD gene in muscle from heterozygotes. Dystrophin was analysed by mixing in increasing proportions (from 0% to 100%) aliquots of solubilised muscle from BMD patients with a qualitatively abnormal dystrophin and a normal male control. The intensity of the abnormal bands, which could be detected starting with 20% of muscle from the BMD patient, increased progressively according to the affected muscle concentration. In five obligate BMD carriers, two dystrophin bands were observed (corresponding to the products from the X bearing the normal and the BMD alleles), even among those with normal serum enzyme activities. Surprisingly, in the four obligate BMD carriers related to patients in whom an additional dystrophin fragment of 250 kd was present (two of them with raised serum enzymes), this band could not be seen, suggesting that the stability or the mechanism responsible for the synthesis of abnormal dystrophin products differs in heterozygotes compared to affected patients.