Are HAP1 cell lines genetically stable (i.e. do they stay haploid)?

We have observed that haploid cell lines will spontaneously diploidise over time. If a gene has been knocked out, diploidisation will not affect knockout status, but to minimize the potential impact of diploidization one of the following approaches is recommended: Work with cells at very low passage. Prepare stocks of low passage cells and revive frequently. Culture knockout and wild type cells in parallel and allow genomes to stabilize. Then use these high passage cells for experimental analyses.