The human immunoglobulin χ locus. Characterization of the partially duplicated L regions

The L regions are parts of the Cχ proximal (p) and distal (d) copies of the human immunoglobulin x locus and are therefore called the Lp and Ld regions. The two regions with their 25 Vχ genes and pseudogenes have now been cloned, thus completing the cloning of the x locus. Lp has been linked to the neighboring Ap and B regions, while Ld was linked to Ad. There is good evidence that at the other side of Ld, i.e. towards the centromere, the end of the locus has been reached. Most of the cloning and linking was achieved by chromosomal walking, employing cosmid and phage λ clones. No such clones could be found for three small gaps. Two of them were closed by a polymerase chain reaction strategy; the third one was characterized by genomic blot hybridization experiments and eventually bridged by a yeast artificial chromosome clone. Early in evolution, a stretch of about 25 kb which comprised three VK genes near the 5′ end of the L region precursor must have been duplicated, such that the later duplication of large parts of the x locus resulted in the appeareance of two very similar three-gene regions in each, Lp and Ld. Two deletions in the central parts of the L regions, on the other hand, must have occurred after the duplication of the locus, since they are found in Lp and Ld in different positions.