Viruslike Particles in Precancerous Hyperplastic Mammary Tissues of C3H and C3Hf Mice2

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Abstract
Viruslike particles in C3H/Crgl mice, seen with the electron microscope, are abundant in hyperplastic mammary tissues that are not neoplastic but known to be preneoplastic. This includes tissue from hyperplastic alveolar nodules and from hyperactive outgrowths of nodules transplanted into gland-free mammary fat pads. Similar hyperplastic tissues from four different C3H sublines presumably lacking the mammary-tumor virus contain abundant viruslike particles indistinguishable from those in C3H/Crgl tissues. Outgrowths from normal transplants, normal hyperplasias (prelactating lobules) of pregnancy, and “noduloids” induced by the administration of estrogen or cortisol typically contain only rare, scattered particles. The data suggest that the viruslike particles are primarily associated with the precancerous mammary hyperplasias; their relationships to inception of tumor and to the mammary-tumor virus remain unknown.