These mice possess loxP sites on either side of exon 1 of the targeted gene. Mice that are homozygous for this allele are viable, fertile, normal in size and do not display any gross physical or behavioral abnormalities. When these mutant mice are bred to mice that express Cre recombinase, resulting offspring will have exon 1 deleted in the cre-expressing tissue(s). When crossed with mice that express Cre recombinase specifically in early B-cell progenitors, this mutant mouse strain may be useful in studies of B-cell maturation. The Jackson Laboratory 2024, used with permission.