Drug resistance is not confined to prokaryotes, nor even to cellular organisms. The base analog azidothymidine (3′-deoxy-3′-azidothymidine, AZT) can inhibit retroviral reverse transcriptase and is used to treat retroviral infections such as infection with the human immunodeficiency virus which causes AIDS. However, mutant viruses arise whose reverse transcriptase is resistant to the drug. Similarly, protease inhibitors can also be used to treat retroviral infections, because they inhibit the cleavage of the polyproteins that these viruses encode into functional proteins. However, mutations can also arise that make the viruses resistant to these drugs. Note that, unlike the case of the rapidly spreading antibiotic resistance in bacteria, the resistance of the retroviruses to these drugs involves mutations that lead to modification of the target of the drug. Unfortunately, mutations occur in RNA viruses at a very high frequency, making this type of drug resistance also a serious problem.