Diffuse oesophageal leiomyomatosis (DL), an inherited smooth muscle proliferation process, has been reported to be associated with Alport syndrome (AS), a familial nephropathy, mainly dominant X-linked inherited, and characterized by ultrastructural changes of the glomerular basement membrane. The COL4A5 gene, encoding the alpha 5 chain of type IV collagen, has been identified as the site of mutations in families with X-linked AS. Recently, a novel alpha 6(IV) collagen chain encoding gene has been mapped closely upstream of COL4A5, and disruption of the 5' end of both genes has been reported in four patients with DL and AS (DL-AS). Here, we report a long-range restriction map around the COL4A6 locus, and show that the COL4A5/COL4A6 deletion observed in seven patients with DL-AS encompasses only the two first exons of COL4A6, with a breakpoint located in the second intron of COL4A6, whose size exceeds 65 kb. Furthermore, we demonstrate that three patients with AS without DL, known to have a deletion of the 5' part of the COL4A5 gene, display a larger deletion in COL4A6. Moreover, a COL4A6 mRNA product was detected by reverse-transcription-polymerase chain reaction in an oesophageal tumour sample of a patient with DL-AS. These results suggest that DL-AS could be caused by an abnormal truncated alpha 6(IV) chain.