Mammary-Tumor Virion Structures in Mouse Milk Fractions2

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Abstract
Loss of mammary tumor virus during purification was investigated. Virions were found in the discard fractions: cream, low-speed pellet, and high-speed supernatant. A new procedure is described for examining cream (with the electron microscope) for the presence of virus particles in this fraction of milk. Mammary tumor virus (B particles) floated with the cream, even when whole milk was layered under a column of buffer through which the cream was washed on centrifugation. After low-speed centrifugation of diluted milk, an abundance of B particles was found in the pellet. Many virions were in aggregates and others adhered to larger structures. These results demonstrate the unusual adhesive and cohesive properties of this virion and help to explain the difficulty in isolating the milk agent.

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