In families with Duchenne/Becker muscular dystrophy, DNA analysis allows direct detection of the sex-linked dystrophy gene mutation. The detection of two alleles (heterozygous) in the region of a deletion in an affected son, excludes the mother having the same deletion. However, it is known that in isolated cases of this disease there is a risk of mosaicism, resulting in genetically different cell lines in the same or different tissues. Because of this consideration, in a subsequent pregnancy, prenatal diagnosis was performed on the mother, who was previously excluded from carrying the deletion based on DNA analysis of blood leukocytes. The examination showed the sex of the foetus to be male, and notably, a deletion identical to that in the ill boy was detected. This indicates that the patient has a germ cell deletion (germline mosaicism).