Original ArticlesPrimary Cilia Are Preserved in Cellular Blue and Atypical Blue Nevi and Lost in Blue Nevus–like MelanomaSheahon, Kathleen M. MD*; Jankowski, Tyler DO*; Yeh, Iwei MD, PhD*,†; North, Jeffrey P. MD*,†; Pincus, Laura B. MD*,†; LeBoit, Philip E. MD*,†; McCalmont, Timothy H. MD*,†; Lang, Ursula E. MD, PhD*,‡Author Information Departments of *Pathology †Dermatology, University of California, San Francisco ‡Department of Pathology, Zuckerberg San Francisco General Hospital, San Francisco, CA Conflicts of Interest and Source of Funding: Supported by a Dermatology Foundation Career Development award. The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Ursula E. Lang, MD, PhD, Department of Pathology, Division of Dermatopathology, University of California, San Francisco, 1701 Divisadero Street, Suite 280, San Francisco, CA 94115 (e-mail: [email protected]). The American Journal of Surgical Pathology: September 2021 - Volume 45 - Issue 9 - p 1205-1212 doi: 10.1097/PAS.0000000000001739 Buy Metrics Abstract Distinguishing cellular blue nevi (CBNs) and atypical CBNs from blue nevus–like melanoma (BNLM) can be diagnostically challenging. Immunohistochemistry may inform the diagnosis in a subset of cases but is not always diagnostic. Further, ancillary molecular testing is expensive and often requires significant tissue to complete. Primary cilia are cell-surface organelles with roles in signal transduction pathways and have been shown to be preserved in conventional melanocytic nevi but lost in melanoma. Immunofluorescence staining of primary cilia can be performed using a single standard-thickness formalin-fixed paraffin-embedded tissue section and has a turnaround time similar to immunohistochemistry. The percentage of tumoral melanocytes retaining a primary cilium is quantified and reported as the ciliation index. In the current study, we explored the utility of the ciliation index in a series of 31 blue nevus–like lesions, including CBNs (12), atypical CBNs (15), and BNLM (4). The average ciliation index for the CBNs was 59±18%, with a median of 60 (range: 28 to 87). The average ciliation index for atypical CBNs was 59±23, with a median of 59 (range: 20 to 93). The average ciliation index for BNLM was 4±3, with a median of 3 (range: 1 to 8). There was no significant difference in ciliation index between the CBN and atypical CBN categories. There was a significant difference between CBN and BNLM and between atypical CBNs and BNLM (P<0.001 for each). Here, we show that ciliation index is a quantitative diagnostic tool useful in the setting of blue nevus–like neoplasms, with benefits including cost and time efficiency. Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.