A recombination allowing MMTV infection of other species has not been observed, no other species exhibits breast cancer with MMTV-like biology. Most importantly, breast cancer in humans behaves differently from MMTV-induced cancer in mice. The spread of MMTV between mice has only been demonstrated to occur through breast milk. Infected mothers produce virus in their breast milk, which is transferred to nursing pups. Virus in the breast milk infects the pup's white blood cells and breast epithelial cells.
Differences also exist in the tissues infected.
The behavior of the virus during pregnancy in mice also differs from human breast cancer. MMTV induced mammary tumors in mice are increased by pregnancy whereas, pregnancy decreases the risk of breast cancer in humans. MMTV would thus have had toundergo changes in its route of transmission, tissues infected and its activation by pregnancy to behave in a manner compatible with human breast cancer biology. Such major changes in the behavior of the virus would be very unlikely. The authors finally separate first Europe and then the rest of the world based on the ranges of the two types of mice.