How do I count the cells in my dish when preparing to transduce them with lentivirus

You have a couple options for determining cell number: One is to predict the number based on the known growth rate of your cells. That is, if you seeded 1x10^6 cells yesterday and your cells have a doubling time of 24 hours, you probably have approximately 2x10^6 cells in your plate. Another option is to prepare an extra plate that will not be transduced. These cells can be lifted at the time of transduction and counted with a hemacytometer like usual. Some people also use the same cell number as what they originally seeded in the plate. This is a safe method for screening with Decode RNAi viral libraries since you want to transduce with a low MOI. However, we wouldn’t recommend this method if you are trying to use a high MOI.