CTSB and osteoarthritis: Supervised clustering of genes grouped based on their known function and segregated by disease stage of osteoarthritis revealed that genes involved in mast cell proliferation and survival (e.g., KIT and IL3RA), protease processing and/or stabilization (e.g., SRGN and CTSB), and Fc receptor subunits (e.g., FCER1A and FCER1G) were significantly upregulated in the synovium of both early- and end-stage osteoarthritis compared to the healthy synovium (Figure 1—figure supplement 2b).