The synthesis of IFNs requires stimulation by viruses or microbial products binding to Toll-like receptors, or chemical inducers. The development of small-molecule modulators is still in its infancy, but the delineation of the responsible signalling pathways has identified many target proteins.

IFNs constitute a large protein family that can be subdivided into three types, binding to different receptors. These receptors initiate signalling by activating a complex signalling cascade regulated at many levels, resulting in a diverse pattern of ISG induction.

ISGs are a diverse group of more than 300 genes, which can have direct antiviral and antitumour functions. These are attractive targets for high-throughput screening for the identification of new modulators of the IFN system.

IFNs were initially investigated for their potential as antivirals, and are now commonly used in anti-HBV (hepatitis B virus) and anti-HCV (hepatitis C virus) therapy. They might also have prophylactic or therapeutic effectiveness in SARS (severe acute respiratory syndrome), influenza or another virus pandemic.

The first FDA approval of an IFN was, however, not for virus infection but for cancer. The mechanisms of antitumour action are incompletely understood. Aberrations of the IFN system are also emerging as important contributors to cancer development.

IFNs also proved effectiveness in relapsing, remitting multiple sclerosis. It is now common practice to initiate IFN-β treatment at the time of diagnosis.

Because of the effectiveness of IFNs in limiting virus replication, reducing tumour cell mass, controlling disease symptoms and prolonging survival, market sales of IFNs approach US$4 billion. As all the effects of IFNs are mediated through ISGs, understanding of the function of these genes might lead to more efficacious antiviral and anti-cancer drugs.