In order to quantify variations in recombination rates across env we used a previously described experimental system where human T cells are transduced with HIV-1 replication-defective vectors pseudotyped with the Vesicular Stomatitis Virus (VSV) envelope [29] . As this system mimics a single cycle of viral infection in which reverse transcription products neither influence cellular survival, nor confer a specific phenotype to the transduced cells, recombinants that were produced during reverse transcription were not subjected to any selection. After cloning of the reverse transcription products in E. coli, the system enabled identification of the recombinants based on the presence of a lacZ reporter gene ( Figure 1 ). Given that known input sequences were used, such an approach enables the accurate and unambiguous localization of the breakpoint position to precise regions bounded by nucleotides that differ between the two parental sequences.