The [2',5'-bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]-3'- spiro-5"-(4"-amino-1",2"-oxathiole-2",2"-dioxide) (TSAO) derivatives of N1-methylhypoxanthine with linkage to the TSAO moiety through the N9 or N7 atom of the hypoxanthine ring (designated TSAO-m1Hx and 7-TSAO-m1Hx, respectively) are potent and selective inhibitors of human immunodeficiency virus type 1 (HIV-1) but not HIV-2 or simian immunodeficiency virus. Their selectivity indices (ratio of cytotoxic concentration to antivirally active concentration) are > 500. This is a > 15-fold increase in therapeutic index, compared with TSAO-adenine. A HIV-1(IIIB) variant selected for resistance to TSAO-m1Hx (designated HIV-1/TSAO-m1Hx) proved to be cross-resistant to the other TSAO-purine derivatives and to the TSAO-pyrimidine derivatives. However, HIV-1/TSAO-m1Hx was highly sensitive to the HIV-1-specific non-nucleoside tetrahydroimidazobenzodiazepinone, nevirapine, pyridinone L697,661, and several HEPT derivatives. The reverse transcriptase (RT) of HIV-1/TSAO-m1Hx shows a single amino acid change (138-Glu to Lys) that is identical to the amino acid change that has recently been observed in several HIV-1/TSAO-pyrimidine mutant strains. Our observations indicate that the TSAO-purines and TSAO-pyrimidines belong to one pharmacological class of HIV-1-specific RT inhibitors that are targeted at the same molecular site of the HIV-1 RT.