There are numerous reports showing that transfection of the ER gene (ESR1) into previously ER-naïve cells, including human mammary epithelial cells, breast and cervical cancer cells, Chinese Hamster ovary cells, rodent fibroblasts and a human osteosarcoma cell line, show an immediate estrogen-induced regression that is associated with a slower time course than is typical for an apoptotic agent [18] (Zajchowski et al., and references therein). Here, ESR1 is linked to osteosarcoma.