Schematic diagram of the orderedIGKandIGLgene rearrangement and SHM during B-cell development. At the pre-B cell stage, after IGH rearrangements have been completed and a μ heavy chain is produced, recombination at the IGK locus occurs between a V_K and a J_K segment. If such rearrangement is functional, the cell can enter the GCs and undergo SHM, with the end result a mutated IgK⁺ memory B or plasma cell (A). However, if the IGK rearrangements at the pre-B stage produce nonfunctional Igκ chains, additional rearrangements can occur in both IGK alleles, either before entering the GC or during the GC reaction (B). We can distinguish between 2 different types of rearrangements. First, if 5′ V_K and 3′ J_K segments remain still available, secondary VJ_K rearrangements can occur, rendering a new (functional) VJ_K rearrangement (B1). Alternatively, deletional rearrangements via Kde can occur, with excision of the C_K region as shown or with deletion of the entire J_K-C_K region through direct V_K to Kde recombination (B2). Such deletions are normally accompanied by rearrangement of the IGL locus in 22q11 to rescue B cells from apoptosis, which can then enter the GCs undergoing affinity maturation via the SHM process (C).