Failure to monitor and detect unsuppressed virus increases the risk of development of resistance to ART. In particular, failure to achieve or maintain virologic suppression for prolonged periods of time may lead to the sequential development of thymidine analogue mutations (TAMs) in patients taking zidovudine (AZT) or stavudine (d4T) as part of ART regimens. The accumulation of 3 or 4 TAMs confers drug resistance across the nucleoside reverse transcriptase inhibitors (NRTI) class of antiretroviral (ARV) drugs and has implications for future ART regimens, particularly in RLS where second line ART options are limited. Stavudine and zidovudine are currently used as part of the first line regimens in Sub-Saharan Africa and other RLS areas.