analysis of the functional relationship between v3 loop and gp120 context with regard to human immunodeficiency virus coreceptor usage using naturally selected sequences and different viral backbones the human immunodeficiency virus type 1 hiv1 gp120 v3 loop plays a predominant role in chemokine receptor usage however other linear and nonlinear gp120 domains are involved in this step of the hiv1 replication cycle at present the functional relationship between v3 and these domains with regard to coreceptor usage is unclear to gain insights into the nature of this relationship in naturally selected viral variants we developed a recombinant strategy based on two different gp120 backbones derived from cxcr4 x4 and ccr5 r5tropic viral strains respectively using this recombinant model system we evaluated the phenotype patterns conferred to chimeric viruses by exogenous v3 loops from reference molecular clones and samples from infected subjects in 13 of 17 recombinants 76 a comparable phenotype was observed independently of the gp120 backbone whereas in a minority of the recombinant viruses 417 24 viral infectivity depended on the gp120 context no case of differential tropism using identical v3 sequence in the two gp120 contexts was observed sitedirected mutagenesis experiments were performed to evaluate the phenotypic impact of specific v3 motifs the data indicate that while the interaction of hiv1 with chemokine receptors is driven by v3 loop and influenced by its evolutionary potential the gp120 context plays a role in influencing the replication competence of the variants suggesting that compensatory mutations occurring at sites other than v3 are necessary in some cases