While our high-coverage shRNA libraries have demonstrated utility in identifying small molecule drug targets 20, 21, genome-wide screening is no longer limited to RNAi-mediated gene knockdown. We recently developed high-coverage shRNA libraries (25 shRNAs/gene) that facilitate pooled genome-wide screening in mammalian cells with dramatically improved hit reliability 16– 19. High-throughput yeast deletion and RNAi-based screening approaches have emerged as powerful alternatives to drug target identification methods that utilize affinity-based chemoproteomics or chemical-genetic expression signatures 9– 13 reviewed in 14, 15. Thus, the development of improved methods for target identification and mechanism elucidation – critical challenges in drug discovery – should facilitate the development of more effective broad spectrum antiviral therapies. Recently, cell-based phenotypic screens of chemical libraries have generated numerous host-targeting broad spectrum antiviral lead compounds with unidentified targets and mechanisms of action 1, 6– 8. However, extensive efforts to develop such drugs have been stymied by various factors, including on-target toxicity and limited in vivo activity 5. Antivirals targeting host cell processes have great potential to demonstrate activity against a range of viruses, reduce the likelihood of mutational resistance, and serve as frontline therapies for rapidly emerging outbreaks of viral disease such as Ebola and influenza 4. The development of effective broad spectrum antiviral therapies remains a highly attractive (but equally challenging) goal in drug discovery. Together, our results highlight the distinct advantages and limitations of each screening method for identifying drug targets and demonstrate the utility of parallel knockdown and knockout screens for comprehensively probing drug activity. Guided by mechanistic insights from both genomic screens, we found that exogenous deoxycytidine markedly reduces GSK983 cytotoxicity but not antiviral activity, providing an attractive novel approach to improve the therapeutic window of DHODH inhibitors against RNA viruses. We show that GSK983 blocks cell proliferation and dengue virus replication by inhibiting the pyrimidine biosynthesis enzyme dihydroorotate dehydrogenase (DHODH). Here we use parallel genome-wide high-coverage shRNA and CRISPR-Cas9 screens to identify the cellular target and mechanism of action of GSK983, a potent broad spectrum antiviral with unexplained cytotoxicity 1– 3. Despite great promise as therapeutics, such drugs remain largely elusive. Broad spectrum antiviral drugs targeting host processes could potentially treat a wide range of viruses while reducing the likelihood of emergent resistance.