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Essays in Biochemistry 



foster mothers had not developed any visible or palpable sign of cancer 

 at the time they nursed the young C57s. Indeed, the first tumor 

 appeared only after the passage of several months. We can therefore 

 be certain that there had been no transfer of cancer cells to the 

 nurslings, since, if there had been, the transferred cells would have 

 erupted into full-grown tumors in a matter of days rather than of 



Fig. 1. Electron micrograph of mouse mammary carcinoma virus as purified by 



convection-current electrophoresis. The drop preparation was shadowed at tan -1 



12° with chromium. Magnification 17,500X (McCarty-Graff). 



months. The conclusion is inescapable; mammary carcinoma in mice, 

 a disease of the adult female, is transmitted from mother to offspring 

 by an infectious agent in the milk. Complete confirmation of this 

 conclusion was provided by the isolation and characterization of the 

 virus in the author's laboratory. 12 The viral bodies pictured in Fig. 1 

 were isolated from high cancer strain milk. The inoculation of these 

 particles is equivalent to the ingestion of "infected" milk. 



Now, Rous had already shown, as far back as 1911, that cell-free 

 extracts of chicken sarcoma were able to reproduce the disease on 

 inoculation into new hosts. 13 The comprehensive implications of Rous 

 and Bittner's discoveries are not widely appreciated, but subsequent 



