Lymphoblastoid cell lines have been established from nine female patients with Duchenne muscular dystrophy who had previously been reported to have chromosome translocations with breakpoints in the Xp21 region. A detailed cytogenetic comparison of prometaphase chromosomes in these cell lines revealed that six of the translocations had X chromosome breakpoints in the sub-band Xp212 and that one further breakpoint could be assigned to either Xp212 or Xp213. These findings confirm and extend previous observations and provide strong evidence for Xp212 as the site of the Duchenne and Becker loci. For the remaining two translocations the simplest explanation for the observed banding pattern is that the X chromosome breakpoint lies a few thousand kilobases away, in the sub-band Xp211. Other explanations which assume breaks in Xp212 combined with complex local chromosome rearrangements are also presented. It is also possible that the altered banding pattern in these two cases is due to the influence of local sequences on the staining or uncoiling properties of the chromatin.