In frame insertion of a cre recombinase gene into the first coding exon followed by an frt-flanked neomycin cassette. A frameshift mutation was also introduced into exon 5. This allele expresses cre recombinase specifically in B lineage cells throughout development. This allele was made using the same construct used for Cd19tm1(cre)Cgn, however CCB ES cells were used for targeting.