A stage of the in vitro transformation of cells. It is characterized by reduced proliferation of the culture, abnormal mitotic figures, detachment of cells from the culture substrate, and the formation of multinucleated or giant cells. During this massive cultural degeneration, a small number of colonies usually, but not always, survive and give rise to a culture with an apparent unlimited in vitro lifespan. This process was first described in human cells following infection with an oncogenic virus (SV40). See also "cell line", "in vitro transformation" and "in vitro senescence".