Only 30% of BLV-infected cattle develop persistent lymphocytosis (PL), a polyclonal expansion of B cells coexpressing high levels of surface IgM, myeloid (CD11b) or T-specific (CD5) markers [14-16] and less than 5% will die from a fatal leukemia, lymphoma or lymphosarcoma [17]. This evidence concerns the gene CD40LG and lymphoma.