The B10.STA62 strain carries the H-2w27 haplotype derived from a wild mouse captured in the vicinity of Ann Arbor, Michigan. Products of two class II loci composing this haplotype, Aα and Aβ, are serologically, biochemically (by tryptic peptide mapping), and functionally indistinguishable from products controlled by the Aα band Aβ /bgenes of the B10.A(5R) strain. In contrast, the polypeptide chain controlled by the third class II locus, Eβ, is different from that controlled by the Eβ /bgene. This Eβ /w27chain lacks an antigenic determinant present on the Eb molecule and carries determinants lacking on the Eb molecule, the Eβ /band Eβ /w27peptide maps differ in at least six peptides, and cytotoxic T cells specific for the Eβ bchains do not react with B10.STA62 target cells. This great difference between the Eβ /band Eβ /w27chains suggests that the corresponding genes have not been derived from one another by a direct mutational conversion; instead, H-2w27 appears to be a recombinant haplotype derived by crossing-over between the AαAβduplex and the Eβlocus. This is the first recombinant discovered separating these class II loci.
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