Immediate early genes (IEGs) are genes which are activated transiently and rapidly in response to a wide variety of cellular stimuli. They represent a standing response mechanism that is activated at the transcription level in the first round of response to stimuli, before any new proteins are synthesized. Thus IEGs are distinct from "late response" genes, which can only be activated later, following the synthesis of early response gene products. Thus IEGs have been called the "gateway to the genomic response". The term can describe viral regulatory proteins that are synthesized following viral infection of a host cell, or cellular proteins that are made immediately following stimulation of a resting cell by extracellular signals.
About 40 cellular IEGs have been identified so far. The earliest known and best characterized include c-fos, c-myc and c-jun, genes that were found to be homologous to retroviraloncogenes. Thus IEGs are well known as early regulators of cell growth and differentiation signals. However, other findings suggest roles for IEGs in many other cellular processes.
In their role as "gateways to genomic response", many IEG products are naturally transcription factors or other DNA-binding proteins. However, other important classes of IEG products include secreted proteins, cytoskeletal proteins, and receptor subunits.