These mutations likely occur in lymphocytes prior to mammary gland transmission since (i) Sag-mediated T-cell deletion is delayed in MMTV-SD-infected BALB/c mice, as we previously showed (9); (ii) endogenous Mtvs capable of recombination with exogenous MMTV are expressed specifically in lymphocytes and not in mammary gland cells (58); and (iii) TBLV replicates at high levels in T cells and does not induce mammary cancer (47, 49). Here, SAG is linked to breast cancer.