A large section of the gene locus encoding human immunoglobulin variable regions of the kappa type is duplicated

The structure of a new segment of the gene locus encoding the variable regions of human immunoglobulins of the Kappa type (VK) has been elucidated. This segment (cluster B) encompasses six VK sequences, which belong to three different subgroups and which are arranged in the same transcriptional orientation. Part of cluster B was found to be very similar to another region of the VK gene locus, which was cloned previously (cluster A). Sequence differences between the homologous region of clusters A and B range from 0.2% to 3.7% depending on the position of the VK sequences. The divergence is in the same range for genes and pseudogenes. Hybridization experiments with DNAs from different individuals clearly demonstrate that the two segments are located at different positions within the VK locus and do not represent allelic variants. The sequence homology between clusters A and B is higher than the homology of both clusters to an allelic variant, which is represented by a DNA segment that had been isolated from another individual. These results, together with a report in the literature of two other homologous regions in the VK locus, make it very likely that a major part of even the whole locus is duplicated. In this case, VK gene numbers would be higher than previously estimated on the basis of hybridization studies. An inverse orientation of VK gene clusters would explain published data on rearrangement products in B-cells if an inversion-deletion mechanism is assumed.