Tumor-inducing properties of the mammary tumor agent in young and adult mice.
The mammary tumor-inducing activity of the mammary tumor agent was studied by the administration of either one or five injections of fractions of transplanted mammary cancer into mice ranging in age from 22 to 126 days.

The concentration of the fractions represented the amount of the agent derived from either 2 × 10-2 or 10-3 gm. equivalents of fresh tissue.

In general, the incidences observed in adult mice could not be correlated with the amount of the agent that was injected.

The highest incidence occurred in the progeny of the mice which received a single injection of the fraction with the lowest concentration of the agent.

The transmission of the agent was observed following the administration of the mammary tumor agent into both young and adult females.

No significant difference was to be seen in the transmission of the agent in the milk of either cancerous or noncancerous experimental mice.

Data were presented suggesting that the age resistance to the tumor-inducing action of the mammary tumor agent may be determined, in part, by the genetic constitution of the test animals. Other possibilities were discussed.
SOME POSSIBLE EFFECTS OF NURSING ON THE MAMMARY GLAND TUMOR INCIDENCE IN MICE.
A preliminary report on some possible effects of nursing on the mammary gland tumor incidence in mice is presented. 3 litters of mice from an inbred strain of mice with a mammary gland tumor incidence of 88% were fostered by a strain of mice with a tumor indicence of about 10%. Of the 9 females fostered by the low tumor strain 3 developed mammary gland tumors 4 had primary lung tumors and 2 died nontumorus. Among 40 of their progeny who were fostered 12 developed breast tumors 2 had breast and pulmonary cancers 13 had primary lung tumors and 13 died nontumorus. 10 of the 13 progeny of the fostered females which had breast cancer developed similar growths as compared with 4 of the 24 progeny from fostered females which had lung tumors. The respective proportions were 77% and 17%. These results support the belief in the extrachromosomal influence as a cause in the development of mammary tumor.
The Effect of Estrogen on the Incidence of Mammary and Pituitary Tumors in Hybrid Mice
To determine if there is a genetic tendency to develop mammary carcinomas mice of a mammary tumor-susceptible strain (CBA) were mated to mice of a mammary tumor-resistant strain (C57). 52 hybrid mice with CBA mothers and 53 hybrid mice with C57 mothers received injections of 16.6 or 50 mcg of estradiol benzoate weekly throughout life. Other mice of similar origin were untreated but were permitted to breed the young being removed at birth. 70% of CBA strain-females and none of the C57 strain-females developed mammary adenocarcinomas. 59% of the estrogen-treated male and female mice with CBA mothers and 85% of the breeding hybrid females of similar origin developed mammary cancers. None of the estrogen-treated hybrid mice with C57 mothers developed mammary tumors although 43% of the breeding females had mammary tumors at death (only 1/3 of the tumors were considered malignant). Continuous pregnancies predisposed to a higher incidence of mammary tumors than estrogen administration. Estrogen-treated hybrid mice with tumor-susceptible CBA maternal parents developed nonmalignant and premalignant localized mammary growths but these were lacking in mice with C57 mothers. Tendency for localized overgrowths was acquired with tendency to develop a high incidence of mammary tumors. Hypophysial tumors were observed in the estrogen-treated hybrids of both groups with the incidence being greater in males than in females. Transmission of the tendency to develop such tumors is compatible with genetic interpretation. Estrogen-treated hybrids had a greater incidence of lymphoid tumors than controls. No histological changes of the adrenal glands could be associated with the tendency to develop mammary tumors.