Fig. 2. Two examples where subcellular localization of signaling modules is essential. Cell fate specification in the worm sex organ and the fly eye occur in epithelia. In each case, an inducing cell (anchor cell or cell R8) provides a ligand (secreted EGF or transmembrane protein BOSS) that interacts with a receptor tyrosine kinase (Let-27 or sevenless) to activate a canonical Ras/MAPK pathway. The MAPK pathway directs a precursor (vulval precursor or Cell R7) toward a particular cell fate. In the case of vulva cell fate specification in C. elegans, the anchor cell is in the stroma, and therefore Let-27, and presumably its downstream targets, must reside in the basolateral compartment to receive the signal. In the fly eye, both the receptor, sevenless and the ligand, BOSS, are located in the microvilli of the apical compartment.