Identifying mutations that cause specific osteochondrodysplasias will provide novel insights into the function of genes that are essential for skeletal morphogenesis. We report here that an autosomal dominant form of Stickler syndrome, characterized by mild spondy-Ioepiphyseal dysplasia, osteoarthritis, and sensorineural hearing loss, but no eye involvement, is caused by a splice donor site mutation resulting in" in-frame" exon skipping within the COLllA2 gene, encoding the a2 (XI) chain of the quantitatively minor fibrillar collagen XI. We also show that an autosomal recessive disorder with similar, but more severe, characteristics is linked to the COL 11A2 locus and is caused by a glycine to arginine substitution in a2 (XI) collagen. The results suggest that mutations in collagen XI genes are associated with a spectrum of abnormalities in human skeletal development and support the conclusion of others, based on studies of murine chondrodysplasia, that collagen XI is essential for skeletal morphogenesis.