Programme Hospitalier Recherche Clinique, Institut Pasteur, Inserm, French Public Health Agency.

Clonal analysis strongly suggests that the two main genetic pathways, 155 and 148, involved in the development of resistance to raltegravir are independent and exclusive. Moreover, the switch of the resistance profile from 155 to 148 may be related to the higher level of resistance to raltegravir conferred by the 148 pathway and also to the higher instability of the 155 pathway.