codon – the word for the junction on genetic material (DNA or RNA) occupied by three nucleotides (or bases) to form an amino acid. In HIV it is most used when refering to drug resistance. There is an amino acid at each codon and the codons are numbered (starting from 1) for each section of the HIV gene. For example the mutation M184V in RT refers to a mutation change at codon 184 in the reverse transcriptase gene.