Abstract: The invention relates to the isolation, purification and characterization of the enzyme xylosyltransferase (defined as “XT”). The invention describes for the first time that XT occurs in at least two isoforms (“XT-I”, “XT-II”). The invention relates furthermore to the recombinant cloning and expression of human and rat XT-I and XT-II and discloses their DNA and protein sequences. The enzymes according to the invention can be used as therapeutic agents and as diagnostic markers, e.g. for the determination of enhanced proteoglycan biosynthesis, and as biochemical markers for determination of several pathological processes such as systemic sclerosis.