E2F and Myc-Induced Lymphoma Figure 5. Analysis of E2F2 cohort lymphomas shows that some E2f2-null Em-myc transgenic tumors are polyclonal. (A) The clonality of lymphomas as indicated by the pattern of immunoglobulin heavy chain rearrangements in Southern blotting. Lymphomas were characterized as monoclonal if there were zero, one, or two bands in addition to the residual genomic band, as biclonal if there were three or four bands in addition to the genomic band, and as oligoclonal if there were five or more bands in addition to the genomic band. (B) Flow cytometric analysis of lymphomas. Top panels, an E2f2+/+ Em-myc transgenic lymphoma and bottom panels, an E2f22/2 Em-myc transgenic lymphoma. The tumor cells were stained to identify mature B cell subsets defined by B220 and CD19 and surface expression of IgM and IgD. doi:10.1371/journal.pgen.1000640.g005 of mixed T cell tumors in their study and concluded that such mixed tumors originated as separate clones and could be expected with a high rate of tumorigenesis [39]. Also consistent with the hypothesis that the enhancement of early onset tumors in E2f22/2 mice is the consequence a larger pool of susceptible cells, rather than of differently behaving cells, is the finding that the lymphomas that emerged were very similar to lymphomas that arose early in E2f2+/+ Em-myc mice. For instance, when mice PLoS Genetics | www.plosgenetics.org 8 September 2009 | Volume 5 | Issue 9 | e1000640