Examination of the carrier state was performed in 744 unrelated mothers of the Duchenne muscular dystrophy/Becker muscular dystrophy (DMD/BMD) probands with identified mutations in the dystrophin gene. Owing to that it was possible to assess frequency and type of new mutations in the gene. Contrary to the Japanese observations of Lee et al. published in this journal, we did not find significant differences in the carrier frequency between mothers of DMD and BMD patients. However, we found that new mutations in patients with deletions were significantly more frequent than in those with duplications and small mutations: of 564 unrelated patients with deletions, 236 (41.8%) carried new mutations, the respective values for duplications and small mutations were 21 of 95 patients (22.1%) and 18 of 85 patients (21.2%)-the differences highly significant (P<0.0001).