As constitutive or inducible loss of MCL-1 impairs the development as well as sustained expansion of many tumors,33, 34, 41 we tested whether the Mcl-1fl allele(s) had been recombined in the lymphomas that arose in the Eμ-Myc;CD19-Cre;Mcl-1fl/+and Eμ-Myc;CD19-Cre;Mcl-1fl/fl mice, or whether selection against Mcl-1 gene loss had occurred during their malignant transformation. This evidence concerns the gene CD19 and lymphoma.