Cell quenching by LN2. Rinsed cover slips containing cells were placed in an open Petri dish, which was placed into a container (e.g., folded by aluminum foil). 10-20 mL of LN2 was carefully poured all over the Petri dish containing coverslips to ensure rapid freezing. Excessive LN2 was cautiously removed by tilting the Petri dish with tweezer. This step must be carried out quickly to prevent formation of large ice crystals due to residual moisture from the earlier washing step.