X-linked sideroblastic anaemia (XLSA; OMIM 301 300) is characterised by accumulation of inorganic iron in erythroblast mitochondria, visualised on staining as distinctive perinuclear rings. It arises from a deficiency of the erythroid specific isoenzyme of δ-aminolaevulinate synthase (ALAS2; E.C. 2.3.1.3.7), caused mainly by mutations affecting the catalytic or substrate-binding domains.^1,2 ALAS2 uses pyridoxal-5-phosphate as a cofactor to catalyse the first, rate-limiting step of erythroid haem synthesis and pyridoxine treatment can alleviate anaemia in many cases, although the response is variable and affected by factors such as mutation, age and iron load.^3,4 …