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At any time during the culture of a cell strain, cells in the culture may become transformed, meaning that they are no longer subject to crisis and senescence but can be passaged indefinitely. i e the transformed cells become immortalized.

Immortal cell cultures are called cell lines, or sometimes continuous cell lines, to distinguish them from primary cultures and cell strains. Eg HEp-2 cells. Immortalization can occur spontaneously during passage of a cell strain, or it can be induced by treatment with chemical mutagens, infection with tumorigenic viruses, or transfection with oncogenes. In addition, cells cultured from tumor tissue frequently readily establish immortal cell lines in culture.

Immortalization is typically accompanied by genetic changes—such cells become aneuploid, containing abnormalities in the number and structure of chromosomes relative to the parent species, and not all cells in a culture of a continuous cell line necessarily display the same karyotype. Like cell strains, cell lines are usually composed of cells that are either fibroblast-like or epithelial-like in morphology.

As with the propagation of cell strains, continued culture of a cell line may result in selection of specific variants that outgrow other cells in the culture over time, and thus with passage the character of a cell line may change substantially, and cell lines of the same origin cultured in different laboratories over a period of years may have significantly different characteristics. It is prudent, therefore, to freeze stocks of cell lines having specific desirable properties so that these cells can be recovered if the properties disappear during culture. Likewise, it makes sense to obtain a cell line showing certain desired characteristics directly from the laboratory that described those characteristics, because cells from alternative sources may differ in character