Various cell models were developed over the years to study long-termestrogen deprivation in estrogen-free environment and using dilution cloning selection(Jiang et al. 1992, Pink et al. 1995, Song et al. 2001, Lewis et al. 2005b).Two breast cancer cell lines were selected after long-term estrogen deprivation (2 years).MCF-7:5C and MCF-7:2A cell lines were at first characterized as ER positive andnon-responsive to estrogens or antiestrogens (Jianget al. 1992, Pink etal. 1995); however, optimization of culture conditions dramaticallyaltered these characteristics (Lewis etal. 2005b). This evidence concerns the gene ESR1 and breast cancer.