MCL-1 appears to be more important than BCL-XL because loss of one Mcl-1 allele substantially delayed lymphoma development in Eμ-Myc;Rag-1-Cre;Mcl-1fl/+ mice, whereas loss of one Bclx allele had only minor impact.29, 30 Loss of BIM-restored lymphoma development in mice with an Eμ-Myc;Bclx−/− lymphoid system and many pre-B/B lymphomas that arose in Eμ-Myc;CD19-Cre;Mcl-1fl/+ or Eμ-Myc;CD19-Cre;Mcl-1fl/fl mice despite loss of one Mcl-1 allele appeared to have undergone selection for low levels of BIM. This evidence concerns the gene RAG1 and lymphoma.