An H-2 haplotype possibly derived by crossing-over between the (A_αA_β) duplex and the E_βlocus

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