mouse mammary tumor virus, an "extrachromosomal factor" (i.e. virus) that could be transmitted between laboratory strains of mice by breast feeding. By the early 1950s, it was known that viruses could remove and incorporate genes and genetic material in cells. It was suggested that such types of viruses could cause cancer by introducing new genes into the genome. Genetic analysis of mice infected with Friend virus confirmed that retroviral integration could disrupt tumor suppressor genes, causing cancer. Viral oncogenes were subsequently discovered and identified to cause cancer. Ludwik Gross identified the first mouse leukemia virus (murine leukemia virus) in 1951 and in 1953 reported on a component of mouse leukemia extract capable of causing solid tumors in mice. This compound was subsequently identified as a virus by Sarah Stewart and Bernice Eddy at the National Cancer Institute, after whom it was once called "SE polyoma". In 1957 Charlotte Friend discovered the Friend virus, a strain of murine leukemia virus capable of causing cancers in immunocompetent mice. Though her findings received significant backlash, they were eventually accepted by the field and cemented the validity of viral oncogenesis. In 1961 Eddy discovered the simian vacuolating virus 40 (SV40). Merck Laboratory also confirmed the existence of a rhesus macaque virus contaminating cells used to make Salk and Sabin polio vaccines. Several years later, it was shown to cause cancer in Syrian hamsters, raising concern about possible human health implications. Scientific consensus now strongly agrees that this is not likely to cause human cancer. History of human oncoviruses In 1964 Anthony Epstein, Bert Achong and Yvonne Barr identified the first human oncovirus from Burkitt's lymphoma cells. A herpesvirus, this virus is formally known as human herpesvirus 4 but more commonly called Epstein–Barr virus or EBV. In the mid-1960s Baruch Blumberg first physically isolated and characterized Hepatitis B while working at the National Institute of Health (NIH) and later the Fox Chase Cancer Center. Although this agent was the clear cause of hepatitis and might contribute to liver cancer hepatocellular carcinoma, this link was not firmly established until epidemiologic studies were performed in the 1980s by R. Palmer Beasley and others. In 1980 the first human retrovirus, Human T-lymphotropic virus 1 (HTLV-I), was discovered by Bernard Poiesz and Robert Gallo at NIH, and independently by Mitsuaki Yoshida and coworkers in Japan. But it was not certain whether HTLV-I promoted leukemia. In 1981 Yorio Hinuma and