ERVW-1 and infection: This approach had the following substantive changes: instead of SupT1 cells, we used SupT1.CCR5 cells (SupT1 cells that express CCR5 in addition to CXCR4 [Boyd et al., 2015]) to support growth of viruses with transmitted-founder, CCR5-tropic Envs; we used more virions for the first passage (≥3×106 versus 5×105 infectious units per library) to avoid bottlenecking library diversity; and rather than performing a full second passage we just did a short high-MOI infection to enable recovery of env genes from infectious virions without bottlenecking (Figure 2A).