Many progenitor cells are also referred to as transit cells,[5][6] sometimes also called transit amplifying cells, the meaning of this term being that the transit cell may found a new sub-lineage but the number of resultant cells is strictly limited (although possibly very large, even trillions yet finite) and the lineage is terminated by cells that die off (by apoptosis) or remain as cells that can no longer divide. Examples of such cells are CFUs (Colony-forming units - referred to as such because of their ability to form colonies in vitro in artificial media) such as CFU-T.[7]