It is important to store cells for future studies. Cryopreservation ensures that you have back-up cells in case of contamination and loss of cell supply. It is best to cryopreserve cells when they are at their maximum growth rate or almost confluent.
Cell banks
It is common practice to create a master bank consisting of 2 to 20 vials of the cell line. Then create one or two working banks from this with 2 to 20 vials in each (depending on how often the cells will be required). When the working bank is used up, a new working bank can be cultured and created from one vial of the original master bank. If possible, keep the master and working bank in separate liquid nitrogen storage tanks.
This will ensure you always have a stock of cells from a lower passage number and it will also not be necessary to keep purchasing the cell line.