ERVW-1 and infection: Inaddition, since the system is based on lentiviral vectors that can only undergo a singleround of cellular infection, the experiments can be safely performed at biosafety level 2.Therefore, compared to prior Env deep mutational scanning that used fully replicativeHIV16,18,29,35, this system both enables more accurate measurements of how Envmutations affect neutralization and is safer and more convenient.