Immunoglobulin class-switching

(A) Before class-switching. A rearranged VDJH gene is at the left, the first five CH genes are shown (and the ε pseudogene). All CH genes, except Cδ, have a 5'-associated sequence called the switch (S) region or switch sequence. These are the sites where DNA is cut during the switching process.
(B) The class-switching intermediate. Two switch sequences are juxtaposed, in this case Sµ and Sγ1. The intervening DNA is excised in a fashion similar to V gene recombination.