Following the discovery that tumor growth could be decreased by selectively blocking receptors found to be uniquely expressed in estrogen-receptor positive forms of breast cancer [17], early efforts began to identify receptors unique to cancer cells that play a role in enhancing tumor invasiveness, growth, angioneogenesis for tumor support, and migration for which treatments could be designed and directed to deliver therapeutic agents or toxins for the elimination of essential supportive functions and the selective destruction of a cancer. This evidence concerns the gene ESR1 and cancer.