SOX9 and histiocytic sarcoma: Interestingly, ORS cells from HS patients also have been found to have increased number of proliferating progenitor cells and loss of quiescent stem cells.55 In addition, Notch signaling is required for SOX9 expression in various cell types.56–58 This may be a link from the γ-secretase complex (a protease complex essential for Notch signaling) mutations identified in some HS familial cases to HS pathology.11 The role of SOX9 in inflammation pathways is less clear and is likely different in different cell types.