Status of nucleolar channel systems in uterine secretions accurately reflects their prevalence—a marker for the window of implantation—in simultaneously obtained endometrial biopsies Fangyin Meng, M.D., Ph.D., a,b Gregory Zapantis, M.D., c Samuel Z. Williams, M.D., Ph.D., b Harry J. Lieman, M.D., b Erkan Buyuk, M.D., b and U. Thomas Meier, Ph.D. a a Department of Anatomy and Structural Biology and b Department of Obstetrics and Gynecology and Women's Health, Albert Einstein College of Medicine, Bronx; and c New York Reproductive Wellness, Jericho, New York Objective: To assess whether nucleolar channel systems (NCSs) can be detected in exfoliated endometrial epithelial cells (EECs) of uter- ine secretions and whether such noninvasively determined NCS status is associated with significant NCS prevalence in simultaneously obtained endometrial biopsies. Design: Prospective study (December 2015–February 2017). Setting: University-affiliated and private fertility clinics. Patient(s): Luteal-phase patients of reproductive age requiring endometrial biopsy for medical indications. Intervention(s): Uterine secretion aspiration before endometrial biopsy. Cells in uterine secretions were spun onto slides and fixed. NCSs were identified and quantified in cells and paraffin-embedded tissue sections by indirect immunofluorescence. Main Outcome Measure(s): Comparison of NCS status of uterine secretions with NCS prevalence in biopsies. Based on NCS detection, uterine secretions were assigned a status of NCS-positive (n ¼ 15) or NCS-negative (n ¼ 7). NCS prevalence in biopsies was expressed as a percentage of NCSs per EECs. Result(s): NCSs can be detected in exfoliated EECs of uterine secretions. Median NCS prevalence in endometrial biopsies from pa- tients with NCS-positive secretions was 41.9% (interquartile range [IQR], 21.1–53.9) versus 2.0% (IQR, 0–6.9) when secretions were NCS-negative. The NCS status of secretions identified a significant difference in NCS prevalence of simultaneously obtained biopsies. Conclusion(s): NCS status of secretions accurately reflects NCS prevalence of biopsies, a marker for the implantation window. As secretion aspiration is compatible with same-day ET, our study provides proof of principle for a minimally invasive approach to determine endometrial receptivity for timing frozen ET. (Fertil Steril Ò 2018;109:165–71. Ó2017 by American Society for Reproductive Medicine.) Key Words: Nucleolar channel system, endometrial receptivity, window of implantation, uterine secretion, noninvasive Discuss: You can discuss this article with its authors and other readers at https://www.fertstertdialog.com/users/16110-fertility- and-sterility/posts/20846-24802. Received August 3, 2017; revised and accepted October 3, 2017; published online November 23, 2017. F.M. has nothing to disclose. G.Z. has nothing to disclose. S.Z.W. has nothing to disclose. H.J.L. has nothing to disclose. E.B. has nothing to disclose. U.T.M. has nothing to disclose. Reprint requests: U. Thomas Meier, Ph.D., Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461 (E-mail: tom.meier@einstein.yu.edu). Fertility and Sterility® Vol. 109, No. 1, January 2018 0015-0282/$36.00 Copyright ©2017 American Society for Reproductive Medicine, Published by Elsevier Inc. https://doi.org/10.1016/j.fertnstert.2017.10.005 VOL. 109 NO. 1 / JANUARY 2018 165 ORIGINAL ARTICLE: REPRODUCTIVE SCIENCE