Given that loss of one allele of Mcl-1 suffices to potently induce cell death in malignant Eμ-Myc lymphomas,35 we hypothesized that loss of one Mcl-1fl allele might also reduce the numbers of pre-leukemic pro-B, pre-B and/or sIg+-B cells (at 3–4 weeks of age) in Eμ-Myc;CD19-Cre;Mcl-1fl/+mice compared with Eμ-Myc and Eμ-Myc;CD19-Cre control animals. This evidence concerns the gene CD19 and lymphoma.