The molecular defect involves the genes coding for RBC membrane proteins ANK1 (ankyrin, 50–60% of HS patients), SPTA1 or SPTB (α- or β-spectrin, 20% of HS, taken together), SLC4A1 (band 3, 15–20% of cases), and EPB42 (protein 4.2), the latter being mutated in autosomal recessive HS, more common in Japan but rarer in other populations (Table 1). The gene discussed is SLC4A1; the disease is histiocytic sarcoma.