Functional Screens to Identify Essential Genes in Human Cells. (A) Two approaches taken by Dr. Tim Wang and coauthors to identify genes essential for human cell survival. One approach utilized CRISPR-mediated gene deletion using pools of small-guide RNAs (sgRNAs) targeting all protein-coding genes (top). They simultaneously utilized retroviral-mediated insertion of gene traps into the coding sequence of all protein-coding genes (bottom). (B) Table depicting the cell types used in similar functional genomic screens to identify essential genes in three recent studies.