The recombinant strain D2.GD was originally typed as I-Ad by serological methods. Indeed, the A alpha and A beta chains of the I-A antigens appear to exhibit normal behavior by the criteria of serology and two dimensional gel analysis. However, the E beta chain encoded by the I-A subregion of this strain, one of the two components of the plasma membrane located I-E antigens produced in D2.GD X A.TFR5)F1 animals, has been demonstrated to be the product of an intragenic recombinational event between E beta genes from the d and b haplotypes. Sequence analysis suggests that the amino-terminal portion of the Eg2 beta chain is derived from the d haplotype and, therefore, that the coding strand for this gene is oriented centromeric leads to telomeric (5' to 3' direction). Finally, these data combined with the data of Rose and Cullen (17) allow the ordering of the genes within the I-A subregion as (H-2K), A alpha, A beta, E beta ... (H-2D).

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