Tumor incidence patterns over the course of 300 days in BALB/cAn and the C.D2-Chr 4 congenic strains (N = 40 mice/strain). (A) Comparisons were made between BALB/c and C.D2-MIA, C.D2-Pnd7A, and C.D2-Pnd7B. (B) Comparisons were made between BALB/c and C.D2-Pnd4A, and C.D2-Pnd4C. The experiments illustrated in (A and B) represent two separate induction studies performed within 6 months of each other. See Table 1 for definition of the intervals of DBA/2 chromatin present in each of the C.D2-Chr 4 congenics. The C.D2-MIA mouse carries at least two resistance alleles at the Pctr1 and Pctr2 loci for plasmacytoma development. The Pnd7A and Pnd4C mice carry at least one resistance allele at the Pctr2 locus for plasmacytoma development. In contrast, the Pnd4A and Pnd7B mice were as susceptible as BALB/c and, therefore, were carrying susceptibility alleles at the Pctr2 locus.