Cells of each species package their genes in a characteristic number of chromosomes. This measurement quantifies the number of T-lymphocytes carrying the CD4 molecules on their cell surface. The normal range for CD4 cell counts is 800 to1500 cells per microliter of blood. As CD4 cell count declines, the risk of developing opportunistic infections increases. DNA is a component of the chromosomes located in the cell nucleus.
Resistance is thought to result mainly from genetic mutation.
In HIV, mutations can change the structure of viral enzymes and proteins so that an antiretroviral agent can no longer interact with the protein to block viral replication. Enzymes are catalysts, meaning they speed up the reactions that occur in organisms, but are not consumed by the reaction. Each codon, the basic coding unit, is a triplet of three nucleotide bases that specifies the sequence of the 20 amino acids used in making proteins. The human genome is defined within 24 chromosomes - the 22 autosomes plus the two sex chromosomes. In HIV, a genotyping assay is performed in order to establish what the sequence is and what mutations are present, which may be associated with drug resistance.