Amino acid sequence analysis of the pFc′ fragment obtained by pepsin digestion of an IgG3; G3m(g) human myeloma protein HER shows it to consist of 112 residues. It starts at position 334 (γ1 numbering), contains eight residues of the Cγ2 region, and the whole Cγ3 domain. Comparison with the sequence of γ1 shows five differences including an extra Met at 397. Each is accountable by a single base substitution. The sequence is identical to that of a G3m(b0) molecule except for the previously noted allotype related Tyr/Phe exchange at position 436. The high degree of homology (95%) among γ-chain subclasses suggests a recent diversification.

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This work was supported by United States Public Health Service Research Grants AM 01431 and AM 02594, The Michael and Helen Schaffer Fund and Grant C98 99-12 from the Norwegian Council for the Sciences and Humanities.

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