Genomic and cDNA probes were used to construct a physical map of the DMD region including the 2.3 Mb DMD gene. FIGE-analysis allows rapid screening of the complete region using only a few probes, detecting deletions or duplications in over 60% of the patients. The technique is especially powerful in the analysis of carrier females. We have found two mutational hotspots; a minor hotspot located proximally and a major hotspot within a large, centrally located intron. Deletions involving this latter intron were studied using 100 kb of cloned DNA sequences. Although breakpoints are spread over the entire region, 5 are clustered within 3 kb. Analysis of over 250 BMB/DMD families has underscored the importance of germinal mosaicism as a major diagnostic pitfall. At least 14% of new mutation cases involve germinal mosaicism and this still is a lower estimate, due to small family sizes. Hence, relatives of apparent new mutation patients should be considered to have high risk, and require appropriate counselling.