Type IX collagen is a nonfibrillar collagen composed of three gene products, alpha 1(IX), alpha 2(IX), and alpha 3(IX). Type IX molecules are localized on the surface of type II-containing fibrils and consist of two arms, a long arm that is crosslinked to type II collagen and a short arm that projects into the perifibrillar space. In hyaline cartilage, the alpha 1(IX) collagen transcript encodes a polypeptide with a large N-terminal globular domain (NC4), whereas in many other tissues an alternative transcript encodes an alpha 1(IX) chain with a truncated NC4 domain. It has been proposed that type IX molecules are involved in the interaction of fibrils with each other or with other components of the extracellular matrix. To test this hypothesis, we have generated a mouse strain lacking both isoforms of the alpha 1(IX) chain. Homozygous mutant mice are viable and show no detectable abnormalities at birth but develop a severe degenerative joint disease resembling human osteoarthritis.