Yilmaz, A. et al. Nat. Cell Biol. 20, 610–619 (2018).

To discover how much something is needed, it is a good strategy to try going without it. Yilmaz et al. applied this approach to characterize genes essential for the maintenance of human pluripotent stem cells (PSCs). They combined two previously characterized tools: a haploid human PSC line and CRISPR-based knockout screening. Along with Cas9, the researchers delivered a library of single guide RNAs (sgRNAs) targeting 18,000 genes with 10 sgRNAs per gene to the cells and assessed cell survival and growth rate, as well as the abundance of sgRNAs over time. This study uncovered essential and growth-restricting genes. Looking at the cellular distribution of the proteins encoded by the essential genes, they found high proportions in nuclear and mitochondrial compartments. The definition of what the authors call the essentialome goes far beyond the known transcription factors and highlights, for example, the role of cell-cycle regulators and DNA-repair genes in PSC growth and identity.