FIGURE 23-19 Synthesis of secreted and membrane IgM. The organization of the µ heavy-chain primary transcript is shown at the top: Cµ4 is the exon encoding the fourth µ constant-region domain; µ_s is a coding sequence unique for secreted IgM; TM1 and TM2 are exons that specify the transmembrane domain of the µ chain. Whether secreted or membrane-bound IgM is made depends on which poly(A) site is selected during processing of the primary transcript. (a) If the upstream poly(A) site is used, the resulting mRNA includes the entire Cµ4 exon and specifies the secreted form of the µ chain. (b) If the downstream poly(A) site is used, a splice donor site in the Cµ4 exon allows splicing to the transmembrane exons, yielding a mRNA that encodes the membrane-bound form of the µ chain. Similar mechanisms generate secreted and membrane-bound forms of other Ig isotypes. SS = signal sequence.