AbstractBiochemical analysis of a lethal recessive mutation affecting chondrogenesis in mice further delineated the mechanism by which this pleiotropic gene acts. Cartilage from late fetuses homozygous for chondrodysplasia (cho/cho) were terminally labeled in vitro with Na235SO4, [14C] glucosamine, and [14C] glucose. Rates of precursor incorporation; tissue contents of uronic acid, amino sugar, and protein; and molecular weight of glycosaminoglycan were determined and found normal compared with control littermate cartilage. Samples from mutants, agitated overnight in distilled water and assayed for loss of incorporated35S, showed between 47 and 77% of control activity. The rinse water from mutant cartilage contained correspondingly more35S than did control. Chromatographic analysis of both tissue‐bound and water‐diffusible macro‐molecules confirmed that glycosaminoglycan and protein backbone were intact. Because the molecular content and rates of precursor incorporation were normal the results suggested that thechogene does not alter net synthesis of glycosaminoglycan. The observation that water facilitates extraction of a glycosaminoglycan‐protein complex of normal molecular weight suggests that the defect in chondrogenesis results from defective interaction of proteoglycan and some other matrix constituent. In view of a previous report that collagen fibrils of mutant cartilage are atypically large and natively banded the site of chondrogenic defect might be the interaction between proteoglycan and c