Similar accumulations of B cells with the same phenotype were found in the bone marrows of IgM+/+ TCL1 mice compared with age-matched IgM–/– TCL1 and IgM–/– mice, confirming that IgM expression is necessary for CLL-like lymphoproliferation in the TCL1-transgenic mouse model (Supplemental Figure 8B). The gene discussed is CD40LG; the disease is B-cell chronic lymphocytic leukemia.