Selections with 3TC, FTC, TDF, ABC/3TC and TDF/FTC did not reveal any significant differences between subtype B NL4-3 viruses in terms of mutations acquired. However, exposure of NL4-3 containing subtype C coding sequences at positions 64/65 to ABC, ATC, d4T, ddI, ABC/FTC, TDF/3TC and d4T/ddI always resulted in the selection of K65R. This result was not obtained with viruses that contained subtype C coding sequences at either positions 64 or 65 but not both. Most importantly, addition of the silent polymorphisms at TAM sites eased the pressure for the preferential selection of K65R in the newly created NL4-3 (C 64/65/70), NL4-3 (C 64/65/70/219) and NL4-3 (C 64/65/70/210/219) viruses. We have also shown that pausing in cell-free RT reactions occurs preferentially at the 64/65 site and that this is entierly template-driven and is not dependent on the origin of the RT enzyme employed.