One of the most fascinating aspects of the immune system is that its components are enormously diversified and capable of interacting with an almost infinite number of antigens. Following the dogma “one gene one protein”, it has been, in fact, difficult to understand how an organism which contains in its genome no more than 2 to 3 × 10 5 genes could produce millions of different immunoglobulin molecules. The answer to this paradox came from molecular biology studies that conclusively demonstrated that the variable (V) regions of immunoglobulin chains are encoded by multiple germline DNA segments which are joined into functional variable regions during B-cell ontogeny. 1 - 4