Complex sentence: HIV-1 RT does not have proof-reading activity, this combined with selective pressure from the drug leads to mutations in reverse transcriptase that make the virus less susceptible to NRTIs and NNRTIs.
Split into simpler sentences: HIV-1 RT does not have proof-reading activity. This, combined with selective pressure from the drug, leads to mutations in reverse transcriptase that make the virus less susceptible to NRTIs and NNRTIs.