To determine the mutations of Southern Chinese with Duchenne and Becker muscular dystrophies (DMD, BMD), we analysed 28 DMD and BMD patients in 24 unrelated families for intragenic deletions and duplications by using cDNA probes covering the entire 14 kb of the dystrophin gene. Deletions were detected in nine unrelated patients (seven patients by probe 8 and two by probe 2b-3). Gene duplications were detected by probe 1-2a in two patients with the duplication bands confirmed in both Hind III and Bgl II digests and by densitometry. A third patient was found to have a junction fragment with Bgl II and a duplication band with Hind III by probe 5b-7. Therefore 50% of the 24 unrelated families were found to have either deletions or duplications. A previously undescribed restriction fragment length polymorphism (RFLP) was found in one family with probe 5b-7 in Bgl II digests which was found to segregate with the disease phenotype. This new RFLP was not detected in over 70 unrelated X chromosomes we have examined so far, and appeared to be "private" for this family. The presence of this new restriction site may or may not be the mutation responsible for the disease phenotype.