FLS, a synovial-specific cell type, also plays an important role in pathological conditions such as osteoarthritis and rheumatoid arthritis.43–45 We identified two major subpopulations of FLS: SPARC related modular calcium binding 2 (SMOC2) + FLS (FLS1) and Signal peptide, cub domain and EGF like domain containing 1(SCUBE1) + FLS (FLS2) (Fig. S3A, B). Here, SCUBE1 is linked to osteoarthritis.