A group of abnormal Hb's with normal a chains, but the non-a chains consist of the N-terminal portion of the ? chain joined to the C-terminal portion of the ß chain, apparently as the result of nonhomologous pairing and crossing over between the genes for ß and ? chains. The major types are Hb LeporeBoston (identical to Hb LeporeWashington), Hb LeporeHollandia, and Hb LeporeBaltimore, which differ in the region of crossing over (?87–ß116, ?22–ß50, and ?50–ß86, respectively). Heterozygotes form about 10% Hb Lepore, normal amounts of Hb A2, and moderately increased amounts of Hb F and usually have mild anemia, microcytosis, and hypochromia; homozygotes form only Hb Lepore and Hb F and have severe anemia.