Vira kræver fænotypisk resistens overfor thymidin analoger gennem kombination af mutationer ved codon 41 og 215 eller ved akkumulering af mindst 4 af de 6 mutationer.
Viruses acquire phenotypic resistance to thymidine analogues through the combination of mutations at codons 41 and 215 or by the accumulation of at least four of the six mutations.