FAS and infection: The Brunello lentiviral library was transduced into the FIXgla-Fas/HEK293 reporter cells, stably expressing Cas9, at a low multiplicity of infection (MOI, 0.3) to ensure that most cells received only one stable integrated sgRNA for genetic perturbation (with a coverage of >500 cells expressing each sgRNA)17 (Fig. 2a).