How are CRISPR screens used for gene function studies?

CRISPR screens systematically knock out genes to identify those affecting phenotypes of interest. Library design includes genome-wide or focused pools of gRNAs (3-6 per gene). Screens can be negative selection (identify genes essential for viability - depleted guides), positive selection (identify genes whose loss confers advantage like drug resistance - enriched guides), or FACS-based (sort by marker then sequence). CRISPRi/a screens use dCas9 fusions for repression or activation. Analysis compares guide abundance pre/post selection using algorithms (MAGeCK, BAGEL). Controls include non-targeting guides and essential gene guides. Validation requires individual gene follow-up. Applications include drug target discovery, synthetic lethality, and functional genomics.