Responsiveness and Progression of Mammary Tumors in High-Cancer-Strain Mice2

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Abstract
The growth curves of 176 mammary tumors in 94 BALB/cf and RIII female mice that delivered litters while bearing tumors were checked for the occurrence of responsiveness to pregnancy. Responsiveness, in the form of a total or partial dependence of growth on the stimuli of pregnancy, was observed in many tumors. However, strain differences have been found. In BALB/cf mice, 19 percent of the tumors was responsive to pregnancy, with a clear prevalence of the partially responsive tumors (12.7%). In RIII mice, 80.3 percent of the tumors was responsive to pregnancies, either totally (39.4%) or partially (40.9%). Usually in BALB/cf tumors, responsiveness was rapidly lost by progression, whereas in RIII tumors it was often maintained for long periods. These data lead to the conclusion that responsiveness is a rule with few exceptions in tumors of the RIII strain, whereas it is apparently an exception to the rule in the BALB/cf strain. Other differences in the growth characteristics of mammary tumors of the same strains were shown in a previous investigation. Possible causes for these strain differences are suggested.