The central portion of the dystrophin gene locus is a preferential site for deletions causing progressive muscular dystrophy of the Duchenne type (DMD). The nucleotide sequence of a deletion junction fragment from a DMD patient was determined, revealing that the proximal breakpoint of the deletion in intron 43 fell within the sequence of a transposon-like element. This segment, belonging to the THE-1 family of human transposable elements, is normally present in a complete form in intron 43 of the dystrophin gene. The deletion mutation was maternally transmitted and eliminated two–thirds of the THE-1 element. Analysis of DNA from additional DMD patients revealed a second deletion with the proximal breakpoint mapping within the same THE-1 element.