Taken together, these observations on the measurement of DNA synthesis before and after treatment, of the content of estrogen receptors in breast tumors, and of blood prolactin levels led to the hope that it would be possible to predict the type of patient likely to respond to treatment with tamoxifen, i.e., to develop what is now referred to a “stratified therapy” (i.e., a re-branding of what was formerly known as “personalized medicine”; Smith, 2012). Here, ESR1 is linked to breast neoplasm.