Additionally, during MMTV infections in mice, both type I IFNs and APOBEC3 seem to participate in viral restriction [62,63]; and during infection in human breast cells, an APOBEC3G-mediated mutation in env is positively selected by increasing of MMTV-like infectivity [64], showing that APOBEC3 cluster is activated during MMTV infection both in mice and human breast cells. The gene discussed is ERVW-1; the disease is infection.