Partial deletions of the dystrophin gene are the predominant genetic lesions in Duchenne (DMD) and Becker (BMD) muscular dystrophies. According to the reading frame hypothesis [1], any deletion disrupting the translational reading frame of the mRNA cannot result in expression of the dystrophin molecule and should lead to severe phenotypes of DMD. In contrast, deletions which maintain the reading frame across the deleted exons may give rise to truncated, semifunctional proteins and milder courses of the disease (i.e. BMD). Among the notable exceptions of this hypothesis are very large "in-frame" deletions by which functionally indispensable domains of the dystrophin molecule have been removed. Here, we report on two DMD patients with large intragenic in-frame deletions. Grossly truncated, but stable dystrophin molecules with preserved C-terminal domains were detected at the sarcolemma on cryosections in both patients. However, dystrophin organization on single-teased muscle fibers revealed disarrangement of the costameric pattern, if compared to normal skeletal muscle fibers. Compared to dystrophin-deficient DMD muscle, expression of chromosome-6-encoded dystrophin-related protein (DRP) was greatly diminished in skeletal muscle of both patients. We show, that loss of more than 50% of dystrophin seems to be deleterious for the protein's function and therefore, the extent of the deletions may have an impact on construction of dystrophin mini genes. Moreover, these findings shed new light on the functional significance of the C-terminal domain of dystrophin. They also suggest a negative correlation between sarcolemmal expression of the dystrophin C-terminus and DRP expression at the sarcolemma.