A novel missense mutation, A1754G, in exon 11 of the erythroid-specific delta-aminolaevulinate synthase gene (ALAS2) was identified in a Japanese male with sideroblastic anaemia. ALAS activity in bone marrow cells of the patient was reduced to 53.3% of the normal control. Consistent with this finding, activity of a bacterially expressed ALAS2 mutant protein harbouring this mutation was 19.5% compared with the normal control, but was increased up to 31.6% by the addition of pyridoxal 5'-phosphate (PLP) in vitro. RFLP analysis with Bsp HI restriction revealed that his mother was a carrier of the mutation. These findings suggest that A1754G mutation was inherited in this family in a manner consistent with X-linked inheritance, and is responsible for sideroblastic anaemia in the patient.