Cell lines have been previously isolated from Eμ-myc transgenic mice, but like other spontaneous tumors isolated from transgenic mice, they are heterogeneous ranging from surface IgM (sIgM) negative progenitor B cells to sIgM+ mature B cells, and can appear as lymphomas or leukemias or plasmacytomas [15], [23]–[25]. This evidence concerns the gene CD40LG and leukemia.