Mitotic recombination in germ cells generated two major histocompatibility complex mutant genes shown to be identical by RNA sequence analysis: Kbm9 and Kbm6.

RNA sequencing represents a major procedural simplification for nucleotide sequence analysis of a transcribed gene. Using newly adapted mRNA and cDNA sequencing procedures, we have sequenced 855 nucleotides of Kbm9 mRNA, corresponding to the codons for the aminoterminal 285 amino acids. The inferred DNA sequence of the Kbm9 gene differs from the parental Kb sequence by single nucleotide alterations in each of codons 116 and 121, resulting in Tyr----Phe and Cys----Arg substitutions, respectively. The Kbm9 sequence is identical to that of another independently arising MHC mutant gene, Kbm6. As both the Kbm9 and Kbm6 genes were generated by recombination between the Kb and Q4 genes, our data indicate that the identical genetic interactions have occurred at least twice. The relatively large extent of identity between Q4 and Kb may be responsible for frequent recombination between the two genes. The parents of the original bm9 mutant mice had five identical mutant offspring, which can be explained by mitotic recombination in the germ cells, producing gonadal mosaicism in the C57BL/6 mother. Thus, mitotic recombination, and not meiotic recombination, appears to be responsible for the formation of at least some of the Kb mutants. Such a mechanism probably plays a major role in the generation of diversity in the major histocompatibility complex.