Messenger RNAs encoding variant forms of both ERα (Bollig and Miksicek, 2000) and ERβ (Lu et al, 1998) have been identified in breast cancers and in breast cancer cell lines and there has been considerable debate over the role of such variants in cancer progression (Balleine et al, 1999; Huang et al, 1999). The gene discussed is ESR1; the disease is breast cancer.