FIGURE 10-21 The Drosophila Dscam gene is processed into a vast number of alternative isoforms. Dscam encodes a cell-surface protein on neurons. The protein (bottom) is composed of ten different immunoglobulin (Ig) domains (ovals), six different fibronectin type III domains (rectangles), one transmembrane domain (yellow), and a C-terminal cytoplasmic domain (dark gray). The fully processed mRNA is shown as rectangles representing each exon, with the length of the rectangle corresponding to the length of the exons, and a green circle representing the 5’ cap. Each mRNA contains one of the 12 Ig2 exons shown in light blue (top), one of the 48 Ig3 exons shown in green, one of the 33 Ig7 exons shown in dark blue, and one of the 2 transmembrane exons shown in yellow. The exons shown in pink are spliced into each of the messages. Thus alternative splicing can generate 12 × 48 × 33 × 2 = 38,016 possible isoforms. See M. R. Sawaya et al., 2008, Cell 134:1007.