Antibody class switching is induced by B-cell activators in the presence of cytokines. The identity of the heavy-chain class to which a B cell is switched is regulated by cytokines and B-cell activators at the level of transcription of unrearranged heavy chain constant genes. Gene-targeting experiments in mice have proved the essential role of these transcripts in switch recombination. Their possible functions are discussed in the context of a model for the mechanisms of class switching.