The transgene was created by combining the Igh-6 enhancer with a minimal SV40 promoter to drive the expression of a cre-ERT mutant fusion protein. The fusion protein consisted of cre recombinase fused to the estrogen binding domain of the human estrogen receptor mutated at amino acid 521 (G521R). Expression in ~ 75% of peripheral blood B lymphocytes was confirmed with an anti-cre antibody. (J:69333)