The use of cell culture systems for biological testing presupposes that changes in the behavior of a cell line are a direct response to an experimental stimulus. But how can you evaluate a reliable correlation between the changes in your cell culture and the experiment you performed? How can you be sure that changes are not resulting from an imbalance within the culture system? In both cases the cells’ response might result in different cell attachment, plating efficiency, recovery or survival from sub-cultivation, death rate and / or rate of progression through the cell cycle.