The management of drug resistance has become part of the management of HIV disease in the treated individual. As two or more nucleoside reverse transcriptase inhibitors (NRTIs) are generally part of each antiretroviral regimen, there is a need to fully understand resistance and cross-resistance within this class of drugs. Broad cross-resistance to NRTIs caused by the group of HIV RT mutations associated with zidovudine and stavudine therapy (thymidine analogue mutations or TAMs) has been well established. The response to tenofovir disoproxil fumarate (TDF) therapy is also limited by certain patterns of TAMs (> or = 3 TAMs with M41L or L210W). The K65R mutation can result from tenofovir DF, abacavir, stavudine, zalcitabine or didanosine therapy. From in vitro phenotypic analysis, the K65R mutation shows no cross-resistance to zidovudine, but low-level resistance to tenofovir and the other NRTIs. Based on clinical cut-offs established for the individual NRTIs, the phenotypic results with K65R suggest full-to-partial drug activity for multiple NRTIs, including tenofovir, against the K65R mutant. Similar to the M184V mutation, the K65R mutation is also associated with reduced in vitro viral replication capacity, hallmarks of which can be demonstrated at the enzymatic level. From cross-sectional genotypic analyses, the K65R mutation and TAMs appear to represent separate patterns of NRTI resistance. Among treatment-naive patients who developed the K65R mutation in clinical trials, successful second line regimens were established. Thus, the K65R mutation appears manageable for the sequencing of treatment regimens in the case of its development.