By means of PCR and direct sequencing, all 51 exons and their neighbouring intronic sequences of the COL4A5 gene were analyzed to detect mutations in 17 members from a Chinese family with X-linked Alport syndrome(XLAS). At the position 2240 in exon 26, a single-base deletion(2240deIC) is found in all male patients, and a heterozygous deletion is found in all female patients, whereas no mutation is found in normal and 80 control individuals. Meanwhile, the corresponding PCR products of female patients are cloned and sequenced to confirm the results. It is concluded that the 2240deIC mutation is the underlying cause of XLAS in this family,not a polymorphism. Furthermore,this single-base deletion mutation in COL4A5 gene is first reported in X-linked Alport syndrome.