It has been estimated that mutations in HIV viral populations occur about once in every 10,000 nucleotides copied by the HIV reverse transcriptase enzyme. Because the genome of HIV-1 is approximately 10,000 nucleotides in length, one such error would occur, on average, every time a viral genome is copied . Each virus produced is slightly different from its predecessor giving rise to a swarm of virus variants or "quasi species." The mechanisms of mutation and recombination, which the virus uses to elude immune responses, are readily used to escape pressure exerted by antiviral drugs, resulting in emergence of drug-resistant virus . The utility of nucleic acid sequencing lies in its ability to identify single based pair mutations within the viral genome. By identifying which mutations have occurred, decisions can be made as to which drug the mutated virus is susceptible to. Monitoring patients' viral load and determination of HIV genotype when the viral load increases allows therapy to be optimized in a timely manner.