ISG15 and infection: To confirm these results in an isogenic system, we also compared an ISG15 CRISPR knock out (KO) cell line (C1 ISG15KO) generated on a WT background to their parental line (healthy control C1), by infecting each with single-cycle HIV-1e- GFP/VSVg and quantifying infectivity by flow cytometry analysis on day 3 post infection (Fig 2C and 2D).