Immunogloblin class switch recombination (CSR) is a regulated process that changes antibody effector functions. Recently, Nambu et al. showed that histone acetylation is induced at switch (S) regions undergoing CSR; however, histone acetylation without accompanying S region transcription is insufficient to attract activation-induced cytidine deaminase (AID), which is required for CSR. They also show that AID can associate with RNA polymerase II. These results support the model that germline transcripts are required to form single-stranded DNA, the AID substrate and further suggest that AID is recruited to S regions by the transcriptional machinery.