What do you mean by essential gene?

An "essential gene" is a gene critical to the viability of the cell. The essentiality of genes in HAP1 cells were determined by gene trapping experiments, as described in this paper: https://www.ncbi.nlm.nih.gov/pubmed/24161985 . Knockout cell lines for some putative essential genes are offered on our website, and are generated on request - although we have successfully knocked out some essential genes to produce viable cells, we cannot guarantee that this will always be the case.