This is a bit of a long shot, but here it goes. I have a CreER mouse embryonic stem cell line with a gene of interest (GOI ) flanked by Cre sites. I would like to study some aspects of the cell line after the knock-out of my GOI. There are two critical bottlenecks in the setup. First, the knockout of my GOI is lethal to ES cells, second I need to synchronize the cell cycle, which I've been told is not possible in ES cells. I am looking for ways to differentiate the ES cells into a cell line which I can synchronize, and first of all check whether knockout of the GOI is lethal. Any suggestions as to which cell type and which protocol I can use?