Interferon-stimulated gene

An interferon-stimulated gene (ISG) is a gene that can be expressed in response to stimulation by interferon. Interferons bind to receptors on the surface of a cell, initiating protein signaling pathways within the cell. This interaction leads to the expression of a subset of genes involved in the innate immune system response. ISGs are commonly expressed in response to viral infection, but also during bacterial infection and in the presence of parasites. 
Expression.
ISGs are genes whose expression can be stimulated by interferon, but may also be stimulated by other pathways. Interferons are a type of protein called a cytokine, which is produced in response to infection. When released, they signal to infected cells and other nearby cells that a pathogen is present. 
This signal is passed from one cell to another by binding of the interferon to a cell surface receptor on a naïve cell. The receptor and interferon are taken inside the cell while bound to initiate expression of ISGs.
Interferon activation of ISGs uses the JAK-STAT signaling pathway to induce transcription of ISGs. ISGs can be divided based on what class of interferon they are activated by: type I, type II, or type III interferon. The protein products of ISGs control pathogen infections.
Specifically, type I and type III interferons are antiviral cytokines, triggering ISGs that combat viral infections. Type I interferons are also involved in bacterial infections; however, they can have both beneficial and harmful effects. The type II interferon class only has one cytokine (IFN-γ), which has some antiviral activity, but is more important in establishing cellular immunity through activating macrophages and promoting major histocompatibility complex (MHC) class II.
All ISG stimulation pathways result in the production of transcription factors. Type I and type III interferons produce a protein complex called ISGF3, which acts as a transcription factor, and binds to a promoter sequence called ISRE (interferon stimulated response element). Type II interferons produce a transcription factor called GAF, which binds to a promoter sequence called GAS. These interactions initiate gene expression. These pathways are also commonly initiated by a Toll-like receptor (TLR) on the cell surface. The number and type of ISGs expressed in response to infection is specific to the infecting pathogen.
Function.
ISGs have a wide range of functions used to combat infection at all stages of a pathogen’s lifestyle. For a viral infection, examples include: prohibiting entry of the virus into uninfected cells, stopping viral replication, and preventing the virus from leaving an infected cell. 
Another ISG function is regulating interferon sensitivity of a cell. The expression of pattern recognition receptors like a TLR or common signaling proteins like those found in the JAK-STAT pathway may be upregulated by interferons, making the cell more sensitive to interferons.