Although BMP5 expression is increased in mesenchymal stem cells from systemic lupus erythematosus (SLE) patients [32], BMP4 and BMP5 expression in synovial tissues from osteoarthritis and RA patients [33], and BMP2 and BMP6 expression in the arthritic synovium from RA and spondyloarthropathy patients [34] were decreased. Here, BMP5 is linked to spondyloarthropathy.