Haplotype analyses of the C.D2-Chr 4 congenics showing the smallest region of overlap among the resistant strains to delineate the interval surrounding the Pctr2 locus. The markers typed and their map positions in centiMorgans (cM) are indicated to the left. The C.D2-MIA strain at N10F15 was solidly resistant (0% incidence) and is likely to be homozygous for two (Pctr1 and Pctr2 ) or more resistance genes from DBA/2, which influence plasmacytomagenesis. The C.D2-Fv1 and C.D2-Pnd7 strains have inherited only one resistance locus (Pctr2 ) from DBA/2. Both C.D2-Pnd7A and Pnd7B were capable of restricting the replication of B-tropic viruses, and both had inherited the same allele of Fv1, yet the two strains differed in their tumor induction profiles (see Fig 2A). The Pnd7B strain was susceptible to plasmacytomagenesis; therefore, this interval does not contain the Pctr2 locus. Open boxes refer to the BALB/cAn allele and darkened boxes, the DBA/2 allele.