This conversion is done by an enzyme called reverse transcriptase. During this conversion process, random mistakes or changes in the virus' genetic structure often occur. These changes are called mutations . HIV reproduces rapidly and makes many mistakes as it replicates. Mutations may change the character of the enzymes that allow HIV to carry out its activities.
Some mutations are harmful to the virus and leave it unable to reproduce.
When this happens, the virus is said to be resistant to those drugs. In contrast, virus that is stopped by anti-HIV drugs is said to be sensitive to those drugs. When drug resistance occurs, a person with HIV will typically experience an increase in viral load. High-level drug resistance is likely to lead to HIV disease progression. Proteins are in turn made up of 20 basic building blocks called amino acids.