Animation: Mechanisms of NRTI Resistance (Primer Nucleoside Unblocking)A mechanism that allows HIV to become resistant to NRTI therapy is known as primer nucleoside unblocking. This animation describes how the drug-resistant form of viral reverse transcriptase enzyme can still incorporate NRTI drugs into synthesized viral DNA; however, the presence of the NRTI does not cause the termination of the viral DNA chain. Instead, drug-resistant reverse transcriptase excises the NRTI analogue, allowing elongation of the viral DNA to continue.