ALAS2 and X-linked sideroblastic anemia 1: The most common cause of CSA is X-linked sideroblastic anemia (XLSA) (OMIM: 300752), which is caused by mutations of the erythroid-specific delta-aminolevulinate synthase gene (5′-aminolevulinate synthase 2; ALAS2) located at Xp 11.21.