Regulation of Dictyostelium pattern formation by CAR3 and CAR4. Dictyostelium grow as single cells and initiate multicellular development in response to nutrient deprivation. Developing cells secrete cAMP which acts as both a chemoattractant and a morphogen. CAR1 is required for aggregation and multicellular mound formation, while CAR2 is required for morphogenesis to the slug stage. CAR4 and CAR3 serve as morphogen receptors to establish the anterior/prestalk (blue) and posterior/prespore (red) axis. In the absence of CAR4 the posterior/prespore zone is expanded and in the absence of CAR3 the anterior/prestalk zone is expanded.