Fig. 2. Domain architectures I: cell-surface proteins, cell-surface receptors and cell-surface proteins with unusual domains. The domain architectures of IgSF proteins discussed in this work are shown as black lines representing their amino acid sequence and coloured symbols representing the domains. The legend for different domain types is at the bottom of Fig. 3. The two parallel, grey lines represent the cell membrane. Parts of proteins above the lines are extracellular, parts below the lines are intracellular. Drosophila proteins are in black, C. elegans proteins are in blue text. GPI, glycosyl-phosphatidylinositol anchor.