The C-FOS protein is the product of c-fos mRNA, a member of a family of immediate early gene (IEG) transcription factors (other members include Jun and Egr-1) also identified as proto-oncogenes. The basal expression of c-fos and other IEGs is typically low but increases relatively quickly and often dramatically in response to changes in cellular activity typically caused by external stimuli, such as metabolic stress or neuronal activation. These IEGs are being used to indicate specific cellular activation including the identification of neurons activated by various growth factors, neuroactive drugs, and correlated changes in behavioral or physiological states. C-FOS immunostaining is thus very useful to study changes in neuronal activity in different brain areas in response to a wide range of stimuli.