Three important aspects of immunoglobulin gene organization and structure have emerged from studies of cloned immunoglobulin kappa chain genes. (i) Multiple variable genes are encoded separately in the genome of both immunoglobulin-producing and uncommitted (embryonic) cells, thereby establishing the evolutionary base for generating immunoglobulin diversity. (ii) These genes exist as many small, closely related families (subgroups) that share close sequence homology largely within their own subgroup. (iii) Comparison of two cloned variable gene segments derived from a single subgroup reveals a feature of their structure that distinguishes them from fixed genes (that is, globin genes) and provides, through extensive surrounding sequence homology, a large target for intergenic recombination. This last observation suggests that a simple recombination mechanism may account for their genetic instability in both germ line and somatic cells.

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