Definition: The red respiratory protein of erythrocytes, consisting of approximately 3.8% heme and 96.2% globin, with a molecular weight of 64,450, which as oxyhemoglobin (HbO2) transports oxygen from the lungs to the tissues where the oxygen is readily released and HbO2 becomes Hb. When Hb is exposed to certain chemicals, its normal respiratory function is blocked; the oxygen in HbO2 is easily displaced by carbon monoxide, thereby resulting in the formation of fairly stable carboxyhemoglobin (HbCO), as in asphyxiation resulting from inhalation of exhaust fumes from gasoline engines. When the iron in Hb is oxidized from the ferrous to ferric state, as in poisoning with nitrates and certain other chemicals, a nonrespiratory compound, methemoglobin (MetHb), is formed. In humans there are at least five kinds of normal Hb: two embryonic Hb's (Hb Gower-1, Hb Gower-2), fetal (Hb F), and two adult types (Hb A, Hb A2). There are two α globin chains containing 141 amino acid residues, and two of another kind (β, γ, ΄, ε, or ζ), each containing 146 amino acid residues in four of the Hb's. Hb Gower-1 has two ζ chains and two ε chains. The production of each kind of globin chain is controlled by a structural gene of similar Greek letter designation; normal individuals are homozygous for the normal allele at each locus. Substitution of one amino acid for another in the polypeptide chain can occur at any codon in any of the five loci and have resulted in the production of many hundreds of abnormal Hb types, most of no known clinical significance. In addition, deletions of one or more amino acid residues are known, as well as gene rearrangements due to unequal crossing over between homologous chromosomes. The Hb types below are the main abnormal types known to be of clinical significance. Newly discovered abnormal Hb types are first assigned a name, usually the location where discovered, and a molecular formula is added when determined. The formula consists of Greek letters to designate the basic chains, with subscript 2 if there are two identical chains; a superscript letter (A if normal for adult Hb, etc.) is added, or the superscript may designate the site of amino acid substitution (numbering amino acid residues from the N-terminus of the polypeptide) and specifying the change, using standard abbreviations for the amino acids. There is an exhaustive listing of variant Hb's in MIM in which a composite numbering system is used.