Different ER positive breast cancer cell lines such as T47D and MCF7 have emerged as in vitro models for proliferation studies in the past (MacIndoe, 1988; Pasqualini et al., 1990; Evans et al., 1993; Billich et al., 2000; Schmitt et al., 2001; Maggiolini et al., 2001, 2002). Here, ESR1 is linked to breast carcinoma.