Stem cell lines are Stem Cells that have been isolated from tissue or blood and held in liquid Culture Medium under conditions designed to support their growth and Proliferation. Under the correct conditions this Proliferation enables substantial expansion of the cell numbers. Following expansion, the stem cell cultures can be harvested, divided into vials and preserved at ultra-low temperatures. This stock of frozen cells is called a cell bank and the freezing process is a crucial stage which enables the cell bank to be stored in a viable and stable state until required. The cells can be thawed and re-cultured for research or therapy. Holding the cells in suspended animation this way also enables extensive quality control and safety testing to be performed before the cells are approved for use. In some cases, such as Embryonic Stem Cells the cultures appear to have the capacity to expand indefinitely, without changing. Such cell cultures are called stem cell lines.