Relation of nursing to the extra-chromosomal theory of breast cancer in mice.

This study investigates the effect of nursing on the incidence of breast tumors in mice. The A inbred high-cancer line was used as control and newborn young of A stock mothers were removed and fostered by females of the low-tumor C57 Black or CBA lines. It was apparent that the fostered mice and the progeny of the noncancerous fostered animals lived considerably longer than did mice from the other 2 classes. Controls had a tumor incidence of 83.6% as compared to only 7.4% of the fostered mice. Little variation was observed in the average breast tumor age of the various classes. This experiment demonstrates that the source of milk exerts a decided influence on the development of breast tumors, since mice nursed by low-tumor stock females show a low percentage of tumors, while mice nursed by high-tumor stock females have a high ratio. In general, if the mother had a breast tumor the incidence among its progeny was nearly as high as for the control stock, while young descended from noncancerous mothers usually have a low ratio. These findings, however, do not entail definite conclusions, since exceptions are rather common.