Type X collagen is a homotrimeric, short chain, nonfibrillar collagen that is expressed exclusively by hypertrophic chondrocytes at the sites of endochondral ossification. The distribution and pattern of expression of the type X collagen gene (COL10A1) suggests that mutations altering the structure and synthesis of the protein may be responsible for causing heritable forms of chondrodysplasia. We investigated whether mutations within the human COL10A1 gene were responsible for causing the disorders achondroplasia, hypochondroplasia, pseudoachondroplasia, and thanatophoric dysplasia, by analyzing the coding regions of the gene by using PCR and the single-stranded conformational polymorphism technique. By this approach, seven sequence changes were identified within and flanking the coding regions of the gene of the affected persons. We demonstrated that six of these sequence changes were not responsible for causing these forms of chondrodysplasia but were polymorphic in nature. The sequence changes were used to demonstrate discordant segregation between the COL10A1 locus and achondroplasia and pseudoachondroplasia, in nuclear families. This lack of segregation suggests that mutations within or near the COL10A1 locus are not responsible for these disorders. The seventh sequence change resulted in a valine-to-methionine substitution in the carboxyl-terminal domain of the molecule and was identified in only two hypochondroplasic individuals from a single family. Segregation analysis in this family was inconclusive, and the significance of this substitution remains uncertain.