Review ArticlesUrothelial Carcinoma In Situ (CIS): New InsightsAkhtar, Mohammed MD, FCAP, FRCPA, FRCPath; Al-Bozom, Issam A. MD, FCAP; Ben Gashir, Mohamed MBChB, FRCPath, PhD; Taha, Noheir M. MBBCh; Rashid, Sameera MD; Al-Nabet, Ajayeb D.M.H. PhDAuthor Information Department of Laboratory Medicine and Pathology, Hamad Medical Corporation, Doha, Qatar The authors have no funding or conflicts of interest to disclose. All figures can be viewed online in color at www.anatomicpathology.com. Reprints: Mohammed Akhtar, MD, FCAP, FRCPA, FRCPath, Department of Laboratory Medicine and Pathology, Hamad Medical Corporation, P.O. Box 3050, Doha, Qatar (e-mail: email@example.com). Advances In Anatomic Pathology: September 2019 - Volume 26 - Issue 5 - p 313-319 doi: 10.1097/PAP.0000000000000239 Buy Metrics Abstract Urothelial carcinoma in situ (CIS) is a high-grade noninvasive malignancy with a high tendency of progression. Although it is typically grouped with other nonmuscle invasive bladder cancers, its higher grade and aggressiveness make it a unique clinical entity. Urothelial CIS is histologically characterized by replacement of the urothelium by cells which fulfill the morphologic criteria of malignancy including nuclear pleomorphism, hyperchromasia, prominent nucleoli, and increased numbers of normal and abnormal mitoses. Urothelial CIS may be categorized as primary when it is not associated with any past or present urothelial carcinoma. It is termed as secondary when there is concomitant or previous urothelial carcinoma in the patient. In recent years detailed molecular studies have provided valuable data for intrinsic molecular subclassification of urothelial carcinoma into 2 broad categories namely luminal and basal types with significant implications for prognosis and therapy. Similar studies on urothelial CIS are limited but have provided crucial insight into the molecular basis of CIS. These studies have revealed that urothelial CIS may also be divided into luminal and basal subtypes, but luminal subtype is much more common. It has also been shown that in many cases, luminal type of urothelial CIS may undergo a class switch to basal type during progression to an invasive carcinoma. Additional studies may be required to confirm and further elaborate these findings. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.