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Type-C particles were isolated From a spontaneous mammary adenocarcinoma (R-35) of Sprague-Dawley rats. The virions were propagated in tumor cell cultures. Inoculation of concentrated cell-free virus and tumor tissue-culture cells intraperitoneally into newborn rats produced mammary adenocarcinomas. Serologically, this virus was unrelated to type-C particles of murine, avian, hamster, and feline leukemia viruses. It had a density of 1.16 g/cm8 and contained 60–70S RNA and RNA-dependent DNA polymerase. Recent evidence of this virus infecting lactating rat mammary-gland cells with oncogenic potentials suggests its etiological relationship to rat mammary cancer. Similar, but clearly not identical, particles were detected in a spontaneous mammary carcinoma of a rhesus monkey. In the tumor cells the virions developed intracytoplasmically (type A) and budded off extracellularly from the cell surface. The isolated virus is being propagated in monkey embryo cells and in human leukocytes (NC-37). The ultramorphology of long-term propagated virus exhibited eccentric nucleoids, thus resembling type-B particles of mouse mammary tumor virus both in its development and extracellular form. It had a density 1.16 g/cm3, contained 60-70S RNA, and demonstrated reverse transscriptase. Serologically, the virus did not cross-react with other oncogenic viruses or simian foamy viruses. Type-B morphology of this virus and its biochemical characteristics of oncogenic viruses suggest its etiological relationship to the monkey breast cancer.
Chopra et al. (1972) studied this question.