In summary, our findings provide evidence that adoption of an “open” Env by R5 viruses in response to the selection pressure for better CD4 usage and infection of CD4low cells represents an early step in the chain of events leading to R5-to-X4 evolution, allowing other selection factors such as virus replication-associated mutational events that are required for tropism switch, but which usually come with costs to viral fitness because of structural constraints, to be manifested. This evidence concerns the gene CD4 and infection.