While normal Runx1fl/fl lymphoid cells permit mono-allelic deletion, primary Eμ-Myc lymphomas showed selection for retention of both alleles and attempts to enforce deletion in vivo led to compensatory expansion of p53null blasts retaining Runx1. Surprisingly, Runx1 could be excised completely from established Eμ-Myc lymphoma cell lines in vitro without obvious effects on cell phenotype. This evidence concerns the gene MYC and lymphoma.