COL9A3 and Arthritis: Together with differential expression of genes with roles in cartilage or arthritis, such as platelet derived growth factor receptor alpha (Pdgfra,29), procollagen C-endopeptidase enhancer 2 (Pcolce2,30) and hyaluronan and proteoglycan link protein 1 (Hapln1,31), we noted reduced expression of three collagen transcripts (Col9a2, Col9a3, Col27a1) in the Grem1+Acan- compared to Grem1-Acan+ populations (Supplementary Fig. 3n).