Methods for inducing non-clonal immortalization of normal epithelial cells by directly targeting the two main senescence barriers encountered by cultured epithelial cells. In human mammary epithelial cells (HMEC), the stress-associated stasis barrier was bypassed and the replicative senescence barrier, a consequence of critically shortened telomeres, was bypassed in post-stasis HMEC. Early passage non-clonal immortalized lines exhibited normal karyotypes. Methods of efficient HMEC immortalization, in the absence of "passenger" genomic errors, should facilitate examination of telomerase regulation, immortalization during human carcinoma progression, and methods for screening for toxic and environmental effect on progression.
STATEMENT OF GOVERNMENTAL SUPPORT
 This invention was made with government support under Grant Nos. CA24844, AG033176, AG040081, CA23074 and CA65662 awarded by the National Institutes of Health, under Grant No. BCRP 060444 awarded by the Department of Defense, and under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in the invention.