A new editorial paper was published in Oncotarget's Volume 14 on July 20, 2023, entitled, “Multiclonality of ER expression in DCIS – Implications for clinical practice and future research.” Estrogen receptor (ER) expression is not routinely evaluated in ductal carcinoma in situ (DCIS). This may be because the prognostic role of ER in DCIS was unclear until the UK/ ANZ DCIS trial in 2021 showed that lack of ER expression in DCIS was associated with a greater than 3-fold risk of ipsilateral recurrence. This was the largest (to date) case-control study nested in a DCIS randomized trial. The meticulous study design eliminated treatment allocation bias as well as treatment-related confounding. This was also the first-ever study to show that ER expression in DCIS is multi-clonal — having very important clinical and research implications. A small proportion of otherwise ER-positive DCIS also contained carcinoma in situ (CIS) duct/s that completely lacked ER expression. “This admixture of clearly ER-positive and ER-negative ducts is not the same as heterogeneity in ER expression and I labeled such DCIS cases as multiclonal DCIS even if just one CIS duct in the entire section lacked ER expression.” In his new editorial, Dr. Mangesh A. Thorat from Guy's and St Thomas' NHS Foundation Trust, Queen Mary University of London and King's College London discusses this study in greater detail, the clinical implications of ER status in DCIS, the potential for avoiding overtreatment and undertreatment based on ER expression, and the importance of simple clinical observations in research. “In summary, ER is a strong and independent prognostic biomarker in DCIS and our novel clonal method is a more accurate method to assess ER status in DCIS.” DOI - https://doi.org/10.18632/oncotarget.28450 Correspondence to - Mangesh A. Thorat - thoratmangesh@gmail.com; m.thorat@qmul.ac.uk Sign up for free Altmetric alerts about this article - https://oncotarget.altmetric.com/details/email_updates?id=10.18632%2Foncotarget.28450 Subscribe for free publication alerts from Oncotarget - https://www.oncotarget.com/subscribe/ Keywords - cancer, ductal carcinoma in situ (DCIS), invasive breast cancer, ER, recurrence, clonality About Oncotarget Oncotarget (a primarily oncology-focused, peer-reviewed, open access journal) aims to maximize research impact through insightful peer-review; eliminate borders between specialties by linking different fields of oncology, cancer research and biomedical sciences; and foster application of basic and clinical science. To learn more about Oncotarget, please visit https://www.oncotarget.com and connect with us: SoundCloud - https://soundcloud.com/oncotarget Facebook - https://www.facebook.com/Oncotarget/ Twitter - https://twitter.com/oncotarget Instagram - https://www.instagram.com/oncotargetjrnl/ YouTube - https://www.youtube.com/@OncotargetJournal LinkedIn - https://www.linkedin.com/company/oncotarget Pinterest - https://www.pinterest.com/oncotarget/ Reddit - https://www.reddit.com/user/Oncotarget/ Media Contact MEDIA@IMPACTJOURNALS.COM 18009220957