The DNA sequence in question is usually found in immune cells and helps trigger the activation and proliferation of the cells. These sequences have also been found in proteins from some other cancer-causing viruses such as Epstein Barr virus and Kaposi's sarcoma virus, but what role this sequence plays in cancer development has remained unexplored.
Katz and colleagues now show that this sequence is contained in the Env protein of MMTV and show that expression of Env in mouse breast cells transforms them. This study puts the spotlight on a potential new mechanism for virus-induced breast cell transformation, which has largely been attributed to the integration of viral DNA into locations that trigger the expression of host cancer-causing genes. An intriguing wrinkle to this study is the previously reported presence of sequences highly homologous to MMTV Env protein in the DNA of as many as 40% of human breast tumors, although a human homologue of MMTV has yet to be found.