Identifying new approaches for preventing breastmilk transmission of HIV-1 is an important research priority. To this end, clinical trials are underway to evaluate the efficacy of HAART (zidovudine, lamivudine, nevirapine) during late pregnancy/lactation versus zidovudine/nevirapine peripartum for prevention of breastmilk HIV-1 transmission. It is important to understand the mechanism of effect of these antiretroviral (ARV) strategies on prevention of breastmilk HIV-1 transmission.