We describe a case of female Becker muscular dystrophy with 45,X/46,X,r(X), carrying an out-of-frame deletion in a nonhot-spot region of the DMD gene. Multiplex polymerase chain reaction did not detect the deletion, because the deleted exons 31-42 comprise a nonhot-spot region, and the product for exon 43 was detected because of the amplification of the DMD gene in the ring X chromosome, affecting 24% of cells. We identified the somatic mutation by assessing relative probe signal intensity for exons 31-43, using a multiple ligation probe amplification assay. This case did not conform to the reading-frame rule. The presence of the ring X chromosome that retains the DMD gene that escapes X inactivation may have contributed some degree of compensation for the dystrophin deficiency. This finding could indicate that the reading-frame rule for correlation of clinical severity with type of deletion may not be applicable in Turner mosaicism. Approximately half of patients with Turner syndrome manifest some degree of chromosomal mosaicism. Multiple ligation probe amplification analysis could be a first-choice method for detecting deletions or duplications in Turner mosaic patients such as female carriers.