WARP is a recently identified extracellular matrix molecule with restricted expression in permanent cartilages and a distinct subset of basement membranes in peripheral nerves, muscle, and the central nervous system vasculature. WARP interacts with perlecan, and we also demonstrate here that WARP binds type VI collagen, suggesting a function in bridging connective tissue structures. To understand the in vivo function of WARP, we generated a WARPdeficient mouse strain. WARP-null mice were healthy, viable, and fertile with no overt abnormalities. Motor function and behavioral testing demonstrated that WARP-null mice exhibited a significantly delayed response to acute painful stimulus and impaired fine motor coordination, although general motor function was not affected, suggesting compromised peripheral nerve function. Immunostaining of WARP-interacting ligands demonstrated that the collagen VI microfibrillar matrix was severely reduced and mislocalized in peripheral nerves of WARP-null mice. Further ultrastructural analysis revealed reduced fibrillar collagen deposition within the peripheral nerve extracellular matrix and abnormal partial fusing of adjacent Schwann cell basement membranes, suggesting an important function for WARP in stabilizing the association of the collagenous interstitial matrix with the Schwann cell basement membrane. In contrast, other WARP-deficient tissues such as articular cartilage, intervertebral discs, and skeletal muscle showed no detectable abnormalities, and basement membranes formed normally. Our data demonstrate that although WARP is not essential for basement membrane formation or musculoskeletal development, it has critical roles in the structure and function of peripheral nerves. (3), and in the present study we identify type VI collagen as a ligand for WARP. WARP has a restricted distribution in developing cartilage tissues, where it is expressed at sites of joint cavitation and articular cartilage formation rather than cartilage structures that will undergo endochondral ossification (3). In adult tissues, WARP is highly restricted to the chondrocyte pericellular matrix in articular cartilage and fibrocartilages, where it colocalizes with perlecan and collagen VI (3). Several of the major basement membrane components have been found in the chondrocyte pericellular matrix, suggesting that this structure may be the functional equivalent of a basement membrane in cartilage tissues (4). Consistent with this hypothesis, recent data from our laboratory have demonstrated that WARP is a component of the basement membrane in a limited subset of tissues including the apical ectodermal ridge, the endomysium surrounding muscle fibers, the vasculature of the central nervous system, and the endoneurium of peripheral nerves (5). The principal components of basement membranes are type IV collagen, laminins, nidogens, and proteoglycans including perlecan; however, the composition, structure, and biological properties of basement membranes can differ considerably between different tissues (6,...