The annexins are a group of homologous proteins that bind phospholipids in the presence of calcium. They may provide a major pathway for communication between cellular membranes and their cytoplasmic environment. Annexins have a characteristic "bivalent" activity in the sense that they can draw two membranes together when activated by calcium. This has led to the hypothesis that certain members of this protein family may initiate contact and fusion between a secretory vesicle membrane and the plasma membrane during the process of exocytosis.