My lab studies the function of Hedgehog proteins and other extracellular signals in morphogenesis (pattern formation) and in injury repair and regeneration (pattern maintenance). We study how the distribution of such signals is regulated in tissues, how cells perceive and respond to distinct concentrations of signals, and how such signaling pathways arose in evolution. We also study the normal roles of such signals in stem-cell physiology and their abnormal roles in the formation and expansion of cancer stem cells.