Interferon is a any of a group of small molecular weight proteins (about 30,000 daltons) secreted by cells in response to the presence of foreign double stranded RNA (e.g., viruses). Interferons then act on infected cells stimulating them to produce a protein which prevents replication of the RNA in these cells. They were discovered by Alick Isaacs and Jean Lindenmann in 1957. Human cells produce at least a dozen different types of interferon. Human interferon is now produced by genetic engineering for therapeutic use, to treat some cancers, hepatitis, and multiple sclerosis.