p11 (S100A10) is a member of the S100 protein family and forms a heterotetrameric complex with annexin 2. p11 has also been found to interact with a diverse set of proteins that includes several ion channels and the serotonin 5-HT1B receptor. Several factors such as dexamethasone, growth factors, nitric oxide and antidepressant therapies regulate the expression of p11. Furthermore, studies using mutant mouse models, RNA interference and antisense constructs have implicated p11 in several biological processes; in particular, there is evidence that p11 is involved in the pathophysiology underlying nociception and depression-like states.