Mural nodules are sometimes seen in borderline and malignant ovarian mucinous tumors. Their pathogenesis is uncertain. It is not entirely clear whether mural nodules evolve through divergent differentiation within a single mucinous neoplasm or represent a collision phenomenon between clonally unrelated tumors. Recent molecular profiling studies support the former view. Using next generation sequencing, a group has recently identified clonal relationship between the mural nodules and corresponding more differentiated mucinous neoplasm. They found identical KRAS mutations in paired mucinous tumors and malignant carcinomatous nodules in 6 of 7 cases. They also found identical CDH1 and p53 mutations in both tumor components in some cases. Reference: Ardakani NM, Giardina T, Amanuel B, Stewart CJ. Molecular profiling reveals a clonal relationship between ovarian mucinous tumors and corresponding mural carcinomatous nodules. Am J Surg Pathol 2017;41:1261-1266. The image shows pancytokeratin immunoreactivity in both the carcinomatous mural nodule as well as the malignant mucinous epithelium (same case as the previous three images). Cytokeratin negativity does not rule out anaplastic carcinoma, as high-grade tumor cells may lose cytokeratin expression. Image courtesy of: Raghavendra Pillappa, MD, Dept. of Pathology, Mayo Clinic, Rochester, Minnesota; Used with permission.