More SOX5-positive cells could be detected in SF (Fig. 2A, middle and Fig. 2B) than in macrophages (Fig. 2A, upper and Fig. 2B), whereas co-localization between TRAP positive cells and SOX5 was only occasionally observed in RA synovium (Fig. 2A, lower and Fig. 2B). Here, SOX5 is linked to rheumatoid arthritis.