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We have isolated two major histocompatibility complex (MHC) class II B genes from the inbred fish strain Xiphophorus maculatus Jp 163 A. We mapped one of these genes, designated here as DXB, to linkage group III, linked to a malic enzyme locus, also syntenic with human and mouse MHC. Comparison of genomic and cDNA clones shows the gene consists of six exons and five introns. The encoded [Beta]1 domain has three amino acids deleted and a cytoplasmic tail nine amino acids longer than in other teleost class II [Beta] chains, more similar to HLA-DRB, clawed frog Xela-F3, and nurse shark Gici-B. Key residues for disulfide bonds, glycosylation, and interaction with [Alpha] chains are conserved. These same features are also present in a swordtail (Xiphophorus helleri) genomic DXB PCR clone. A second type of class II B clone was amplified by PCR from X. maculatus and found to be orthologous to class II genes identified in other fishes. This DAB-like gene is 63% identical to the X. maculatus DXB sequence in the conserved [Beta]2-encoding exon and was mapped to new unassigned linkage group LG U24. The DXB gene, then, represents an unlinked duplicated locus not previously identified in teleosts.

Nucleotide excision repair proteins have been implicated in genetic recombination by experiments in Saccharomyces cerevisiae and Drosophila melanogaster, but their role, if any, in mammalian cells is undefined. To investigate the role of the nucleotide excision repair gene ERCC1, the hamster homologue to the S. cerevisiae RADIO gene, we disabled the gene by targeted knockout. Partial tandem duplications of the adenine phosphoribosyltransferase (APRT) gene then were constructed at the endogenous APRT locus in [ERCC1.sup.-] and [ERCC1.sup.+] cells. To detect the full spectrum of gene-altering events, we used a loss-of-function assay in which the parental [APRT.sup.+] tandem duplication could give rise to [APRT.sup.-] cells by homologous recombination, gene rearrangement, or point mutation. Measurement of rates and analysis of individual [APRT.sup.-] products indicated that gene rearrangements (principally deletions) were increased at least 50-fold, whereas homologous recombination was affected little. The formation of deletions is not caused by a general effect of the ERCC1 deficiency on gene stability, because [ERCC1.sup.-] cell lines with a single wild-type copy of the APRT gene yielded no increase in deletions. Thus, deletion formation is dependent on the tandem duplication, and presumably the process of homologous recombination. Recombination-dependent deletion formation in [ERCC1.sup.-] cells is supported by a significant decrease in a particular class of crossover products that are thought to arise by repair of a heteroduplex intermediate in recombination. We suggest that the ERCC1 gene product in mammalian cells is involved in the processing of heteroduplex intermediates in recombination and that the misprocessed intermediates in [ERCC1.sup.-] cells are repaired by illegitimate recombination.