Figure 1.  A comparison of the structure of dTMP, AZTMP, and 3TCMP.  On the left is the normal nucleoside monophosphate, dTMP; in the center, the nucleoside analog (NRTI) AZTMP; and on the right, the analog 3TCTP. Resistance to an NRTI implies that the resistant RT has an enhanced ability to discriminate between the analog and the normal nucleoside.  This enhanced discrimination either can occur at the time the triphosphate form of the NRTI is incorporated into DNA, which is the mechanism of resistance to 3TC caused by the M184V mutation, or can involve enhanced excision of the NRTI after it has been incorporated into DNA by RT, which is the mechanism of AZT resistance associated with mutations M41L, D67N, K70R, T215Y, and F/K219E/Q.