ESR1 and breast cancer: Contributing to the lack of such historical data is the relatively recent development of most currently used assays: in the case of breast cancer, for example, one of the first such assays, for ER, became available only in the 1970s,12 and characterization of molecular phenotypes is an innovation of the 21st century CE.13 We accordingly conducted a novel feasibility study, including assessment of test–retest reliability, for a series of breast cancer cases spanning 6 decades (1947–2009).