The Liaison of Isotype Class Switch and Mismatch Repair: An Illegitimate Affair

The Ig class switch allows the expression of a V region with new C_H regions associated with various effector functions (for reviews, see references 1 and 2). Switch recombination (SR) occurs by an intrachromosomal deletion process in which the intervening genetic material between the switch (S) regions is excised as a circle. The recombination breakpoints are located within S regions but not at consensus recombination signal sequences, suggesting that SR is not site-specific recombination 3. Switch junctions do not contain long stretches of homology, arguing that homologous recombination does not contribute to this process. Thus, SR is unique in that it does not fall neatly into one of the well-defined categories of recombination. Results indicating an important role for mismatch repair (MMR) proteins in Ig class switch, published in a recent issue of EMBO (European Molecular...