The MB1 gene, CD79a, is critical in the immune system. It encodes the Ig-α protein, a key component of the B cell antigen receptor complex. Mb1-iCre mice were generated by integrating a codon-optimized Cre recombinase gene (iCre) expression cassette, which has higher recombination activity, into the endogenous Cd79a gene in the mouse, driving its specific expression. This strategy disrupts the expression of the mouse endogenous Cd79a gene. When bred with mice containing the loxP site-flanked sequence, Cre recombinase-mediated deletionof the flanked sequence is estimated to occur in the lymphoid B cells of the offspring mice, with a predicted high level of Cre recombinase activity in the early development of B cells.