The deduced 418-amino acid human protein contains a signal sequence, followed by a VA domain, 2 fibronectin type III (F3) repeats, and a short pro/arg-rich C terminus. The VA domain has a putative metal ion-dependent adhesion site (MIDAS) motif and 2 potential O-linked glycosylation sites, and each F3 domain has a putative N-glycosylation site. Human WARP shares 79% amino acid identity with mouse Warp. RT-PCR of mRNA from various mouse tissues and cell lines detected Warp expression only in chondrocytes. Northern blot analysis confirmed expression of Warp in chondrocytes isolated from newborn mouse rib cartilage. Western blot analysis of transfected human embryonic kidney cells detected a 48-kD protein in both cell layer and media fractions.