Ah, I was looking at the other paper so haven't inspected the one you linked to. The two papers used the same method though: "The HIV-1 LAI stock was produced

Ah, I was looking at the other paper so haven't inspected the one you linked to. The two papers used the same method though: "The HIV-1 LAI stock was produced by transfection of 293T cells with the pLAI molecular clone." "HIV-1 was first produced by transfecting HEK293T cells with HIV-1 DNA" Sounds like they produced a bunch of virus in 293T cells, during which time it could mutate.