These observations led Szabo et al. [11] to the idea that the MMTV-like sequence-positive kitten was comparable to a suckling mouse, being at the end of the first phase of infection by MMTV, when the virus spreads from Peyer’s patches to other organs, or in the second phase of infection, when all lymphoid organs are involved in a Sag-dependent immune response. This evidence concerns the gene SAG and infection.