The objectives of the current study were: to characterize the OP-1 present in human SF; to compare levels of endogenous OP-1 protein in SF obtained from organ donors, from OA patients, and from RA patients using a validated ELISA [19]; and to correlate these levels with those of other validated biochemical markers of joint tissue metabolism, specifically AgKS [20] and HA [21]. Here, BMP7 is linked to rheumatoid arthritis.