The loss of replicative potential with each cell division has been attributed to the progressive shortening of telomeres. This "mitotic clock" occurs because most normal human cells are telomerase-negative. Telomerase is a multicomponent enzyme that prevents loss of telomeric DNA associated with normal cell division. Transfection of cells with vectors expressing the catalytic subunit of human telomerase (hTERT) is often sufficient for immortalization. In this article, we will address this approach in the establishment of immortalized endometrial cells and its value in facilitating in vitro studies.