- High-level resistance to (-) enantiomeric 2'-deoxy-3'-thiacytidine in vitro is due to one amino acid substitution in the catalytic site of human immunodeficiency virus type 1 reverse transcriptase. Boucher, C.A., Cammack, N., Schipper, P., Schuurman, R., Rouse, P., Wainberg, M.A., Cameron, J.M. Antimicrob. Agents Chemother. (1993)