Hereditary spherocytosis (HS) is the most common inherited haemolytic anaemia in Northern Europeans. The primary molecular defects reside in the red blood cell (RBC) membrane, particularly in proteins that link the membrane skeleton to the overlying lipid bilayer and its integral membrane constituents¹. Ankyrin-1 is the predominant linker molecule. It attaches spectrin, the major skeletal protein, to the cytoplasmic domain of band 3, the RBC anion exchanger. Two-thirds of patients with HS have combined spectrin and ankyrin-1 deficiency^2,3; deficiency of band 3 occurs in about 15 to 20% (ref. 1). These data suggest that ankyrin-1 or band 3 defects may be common in HS. To test this we screened all 42 coding exons plus the 5′ untranslated/promoter region of ankyrin-1 and the 19 coding exons of band 3 in 46 HS families. Twelve ankyrin-1 mutations and five band 3 mutations were identified. Missense mutations and a mutation in the putative ankyrin-1 promoter were common in recessive HS. In contrast, ankyrin-1 and band 3 frameshift and nonsense null mutations prevailed in dominant HS. Increased accu-mulation of the normal protein product partially compensated for the ankyrin-1 or band 3 defects in some of these null mutations. Our findings indi-cate that ankyrin-1 mutations are a major cause of dominant and recessive HS (∼35 to 65%), that band 3 mutations are less common (∼15 to 25%), and that the severity of HS is modified by factors other than the primary gene defect.