Combinations of two nucleoside analogue RT inhibitors (NRTI) are by far the most commonly utilized backbone of combination antiretroviral regimens upon which additional third or fourth agents, a non-nucleoside RT inhibitor (NNRTI) or a (boosted) protease inhibitor (PI), confer sufficient potency for long-term efficacy. The choice of the specific two NRTI is made on the basis of potency and durability, short-and long-term toxicities, drug-drug interactions, the propensity to select for resistance mutations, and dosing convenience. Currently, the most frequently used NRTI combination is zidovudine plus lamivudine, often co-formulated as Combivir®.