Recombination between plt and 8 locus markers on chromosome 4 were examined in 101 {DDD/1-plt/plt × (DDD/1-plt/plt × MSM-+/+)F₁}BC and 89 {(DDD/1-plt/plt × MSM-+/+)F₁ × DDD/1-plt/plt}BC progenies. Phenotypes were assessed with cell count and flow cytometry of PLN, and immunohistochemistry of spleen.plt/plt was distinguished with paucity of PLN cell number (less than 1 × 10⁷) and small content of L-selectin⁺ TCR⁺ cells (less than 31%) (Fig6B). Genomic DNA from BC progeny were amplified by PCR with MapPairs primer (Genetic Research Inc). To detect polymorphism in CD72 gene, the following primers were used, left: 5′-ATGGGAGATGCTGGATGGAGAT-3′ and right: 5′-CAGACCTAATTCCAACACTCAG-3′. The amplified products were subjected to electrophoresis, visualized with ethidium bromide staining, and photographed. The size of the PCR product was compared with that from parental strains to determine its genotype. Recombinant had wild-type phenotype (plt/+) and DDD/1/DDD/1 homozygote genotype, or mutant phenotype (plt/plt) and DDD/1/MSM heterozygote genotype for each locus. Recombination frequency was calculated with Map Manager software created by Drs Kenneth F. Manly and Robert Cudmore (Roswell Park Cancer Institute, State University of New York, Buffalo, NY).