Different classes of antibodies serve different effector functions and therefore the production of different classes must be regulated during an immune response. Numerous recent studies support the accessibility model for the regulation of heavy chain class switch recombination, as heavy chain (CH) genes to which cells will switch have been shown to be hypomethylated and to be trarrscriptionally active (reviewed in Esser and Radbruch, 1990). Each of the unrearranged CH genes has been shown to be transcribed prior to switch recombination to that particular CH gene. The structures of all the CH gene germline transcripts are identical, i.e. they are initiated 5’ to the switch (S) regions and are transcribed in the same direction as and terminate near the same poly(A) sites as mature Ig mRNAs (Fig. 1). An exon 5’ to the S regions (I or germline exon) is spliced to the normal acceptor site for mature mRNAs. The germline RNAs tend to have multiple initiation sites probably due to the fact that they do not have TATA boxes and at least one (germline α RNA) has multiple splice donor sites for the I exon.