Recently, recurrent oncogenic mutations of components of the MAPK pathway, including KRAS, SMAD4, and MAP2K1 (but not BRAF), have been reported in a few cases of, particularly extranodal, Rosai-Dorfman disease (RDD) [11, 13, 14], challenging its assumed non-neoplastic nature in extranodal presentation. Here, MAP2K1 is linked to sinus histiocytosis with massive lymphadenopathy.