ESR1 and breast cancer: Both ERα+ and ERα− disease is found in these cases.62 Similarly, while tamoxifen is reported to reduce development of noninvasive breast cancer (ductal carcinoma in situ (DCIS)), it does not absolutely prevent it.74 In fact, some estimate as many as 40% of tamoxifen-treated breast cancer patients relapse.75 These observations are not inconsistent with the prevalence of preexisting and acquired tamoxifen resistance in breast cancers.