Freshly isolated cultures from mammalian tissues are known as primary cultures until sub-cultured. At this stage, cells are usually heterogeneous but still closely represent the parent cell types as well as in the expression of tissue specific properties. After several sub-cultures onto fresh media, the cell line will either die out or transform to become a continuous cell line. Such cell lines show many alterations from the primary cultures including change in morphology, chromosomal variation and increase in capacity to give rise to tumors in hosts with week immune systems.