Mcs5a acts in a non-mammary cell-autonomous fashion through cells of the immune system. (a) Percentage of transplant sites with one or more carcinomas in the mammary gland transplantation assay. Transplant groups are indicated as donor/recipient. Logistic regression analysis indicated a significant effect (P = 0.04) of the recipient on transplant site carcinoma incidence (Table 1). The animals in susceptible/susceptible (S:S), susceptible/resistant (S:R), resistant/susceptible (R:S), and resistant/resistant (R:R) transplant groups numbered 31, 72, 22, and 88, respectively. (b) 7,12-Dimethylbenz(a)anthracene-induced mammary carcinoma multiplicities (expressed as average ± standard error of the mean) after bone marrow transplantation. Transplant groups are indicated as donor/recipient. Transplant group S:R (carrying an average immune system replacement of 53%; n = 16) displays a mammary carcinoma multiplicity intermediate to transplant groups R:R (n = 17) and S:S (n = 12). Mcs5a, mammary carcinoma susceptibility locus 5a.