Mcs5a acts beyond mammary epithelial cell transformation during early cancer progression. (a) 7,12-Dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma multiplicities (susc., n = 27; Mcs5a, n = 49). (b) N-nitroso-N-methylurea (NMU)-induced mammary carcinoma multiplicities (susc., n = 29; Mcs5a, n = 28). (c) Mammary carcinoma multiplicities induced by mammary ductal infusion of retrovirus expressing the activated HER2/neu oncogene (HER2/neu) (susc., n = 15; Mcs5a, n = 15). (d) HER2/neu-induced in situ carcinoma multiplicities (susc., n = 24; Mcs5a, n = 24). Values are expressed as average ± standard error of the mean. Significance (P < 0.05) is indicated by an asterisk. Mcs5a, resistant congenic harboring the mammary carcinoma susceptibility locus 5a resistance allele; susc., susceptible congenic control.