Resistance mutation

The term resistance mutation is most commonly used to describe point mutations in virus genes that allow the virus to become resistant to treatment with a particular antiviral drug. The term is now being seen with more frequency in bacteriology and parasitology. The term "resistance mutation" first appeared in the management of HIV, which was the first virus in which genome sequencing is routinely used to look for drug resistance.

Nomenclature
Resistance mutations are conventionally listed as a letter, number and letter. For example, the M184V mutation in the reverse transcriptase gene of HIV confers resistance to the drug lamivudine. The letters stand for amino acids and use the traditional one letter abbreviations. M stands for methionine, and V stands for valine; 184 is the number of that amino acid position counting from the carboxy terminus of the protein. M184V means that the 184th amino acid of the protein is normally methionine, but that a mutation in the gene for that protein produces a form of the protein where that amino acid is substituted by valine instead.

The form V184 is sometimes seen; this means that the 184th amino acid of that protein is valine, but it gives no information as to whether that is a mutation or the normal state of affairs. The Greek capital letter delta Δ indicates a deletion: so ΔM184 would mean that the 184th amino acid is usually methionine, but that this position is now missing.

Alternatives are separated by slashes; so, M184V/I means "either M184V, or M184I".