Nucleoside analogues are molecules that act like nucleosides in DNA synthesis. They include a range of antiviral products used to prevent viral replication in infected cells. Nucleoside analogues can be used against hepatitis B virus, hepatitis C virus, herpes simplex, and HIV. Once they are phosphorylated, they work as antimetabolites by being similar enough to nucleotidesto be incorporated into growing DNA strands. Less selective nucleoside analogues are used as chemotherapy agents to treat cancer, eg gemcitabine and 5-FU. Antimetabolite is a chemical that inhibits the use of a metabolite, which is another chemical that is part of normal metabolism. Such substances are often similar in structure to the metabolite that they interfere with, such as the antifolates that interfere with the use of folic acid. The presence of antimetabolites can have toxic effects on cells, such as halting cell growth and cell division, so these compounds are used as chemotherapy for cancer.