['To examine the effect of in-frame deletions in human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) on plasma viremia and phenotypic resistance to antiretroviral drugs.', 'Plasma HIV-1 RNA was isolated from 168 antiretroviral therapy-experienced subjects for quantification of plasma viremia, RT sequence analysis, and phenotypic resistance assays.', 'Four patients were found to harbor HIV-1 strains possessing in-frame, 3-nucleotide deletions at RT codons 67, 69, and 70. In these subjects, phenotypic resistance and high plasma viremia were observed only in a background of multiple resistance mutations. A recombinant virus engineered with an in-frame deletion of RT codon 67 did not have increased resistance to nucleoside reverse transcriptase inhibitors (NRTIs).'] Are deletions in the beta3-beta4 hairpin loop of HIV-1 reverse transcriptase observed in HIV-1 isolated from subjects during long-term antiretroviral therapy?