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Search for retrovirus-like particles in human breast cancer cells in culture.

We have recently established four new human breast cancer cell lines that were characterized as being of human mammary origin. We examined these cell lines for particles morphologically resembling retroviruses by electron microscopy, for extracellular and intracellular particles containing high-molecular-weight RNA and RNA-directed DNA polymerase by biochemical assays, and for mouse mammary tumor virus (MMTV)-related sequences in the cell genomes by molecular hybridization. An extensive search for budding particles by thin-section electron microscopy of cells did not provide evidence for retrovirus-like particles. Similarly, 1000- to 2000-fold concentrated samples of medium harvested from 10(8) cells did not contain particles of a density of 1.14 to 1.16 g/ml containing RNA-directed DNA polymerase. Compared with DNA polymerase activity of MMTV, and taking into account the particle weight and protein content of retroviruses, we estimate that, if these cells produce retrovirus-like particles, this production would be less than 1.6 particles/cell every 24 to 72 hr. The hybridization of cell DNA with MMTV complementary DNA also did not show detectable amounts of virus-related sequences in the cell genome. Analysis of the hybridization results suggested that, if the human breast cells contained MMTV-related sequences, they must be present in less than one copy per 100 cells. Thus, we have obtained no convincing evidence for the presence of retrovirus-like particles or subviral components in these cells. It is of course possible that these cells contain virus information but at levels below the sensitivity of our assay procedures.