Many mammalian cells have indentations in the plasma membrane called caveolae, but exactly what these flask-shaped pits do remains unclear. In a Perspective, Parton discusses the characteristics of caveolin-1-deficient mice that lack caveolae ( Drab et al.). The mice are fertile and seem healthy but show cardiovascular defects, suggesting that caveolae may be important in nitric oxide signaling and cellular proliferation.