Critical to our understanding of the immune system diversity is the determination of the number of germ line V genes. The total number of V genes is given by the product: number of subgroups x number of germ line genes per subgroup. Studies of K chains and of embryonic DNA indicate 5-10 V genes per subgroup. Statistical analysis of the limited sequence data of mouse K chains suggest about 50 VK subgroups. We report here a general approach for direct estimation of the number of VL and VH subgroups expressed in normal spleen, and present data for VK. The K mRNA of the spleen is a heterogeneous population where different Vk are linked to the same CK, i.e. CK equals total VK. The ratio CK/distinct VK approximates the number of subgroups since VK of the same subgroup cross hybridize while VK of different subgroups do not. This ratio was determined by molecular hybridization of cloned CK and VK DNA probes with spleen mRNA. The results indicate the expression of 280 VK subgroups in mouse spleen. Assuming an average of 7 genes per subgroup, we estimate about 2000 VK germ line genes.
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