Progeroid type Ehlers-Danlos (E-D) syndrome was reported to be caused by defects in galactosyltransferase I (EC 2.4.1.133), which is involved in the synthesis of common linkage regions of proteoglycans. Recently, we isolated cDNA of the galactosyltransferase I (XGalT-1) (Okajima, T., Yoshida, K., Kondo, T., and Furukawa, K. (1999) J. Biol. Chem. 274, 22915-22918). Therefore, we analyzed mutations in this gene of a patient with progeroid type E-D syndrome by reverse transcription polymerase chain reaction and direct sequencing. Two changes of G and T to A and C at 186 and 206, respectively, were detected. Then, we determined the genomic DNA sequences encompassing the A186D and L206P mutations, revealing that the unaffected parents and two siblings were heterozygous for either one of the two different mutations and normal, while the patient had both of two different mutant genes. Enzymatic functions of cDNA clones of XGalT-1 containing the individual mutations were examined, elucidating that L206P clone completely lost the activity, while A186D retained approximately 50% or 10% of the activity when analyzed with extracts from cDNA transfectant cells or recombinant soluble enzymes, respectively. Moreover, L206P enzyme showed diffuse staining in the cytoplasm of transfectant cells, while the wild type or A186D clones showed Golgi pattern. These results indicated that the mutations in XGalT-1 were at least one of main molecular basis for progeroid type E-D syndrome.