Xylosyltransferases (XTs) are ubiquitously expressed enzymes that belong to the group of glycosyltransferases. These enzymes initiate the proteoglycans formation by assembly of glycosaminoglycan (GAG) chains to the protein substrate by catalysing the transfer of xylose from the nucleoside diphosphate donor (UDP-xylose) to serine residues in the protein [1, 2]. XTs reside in the endoplasmatic reticulum and/or cis-Golgi, but they are also released into extracellular matrix . The XTs serum activity has been proposed as biomarker of altered proteoglycan metabolism in various diseases, eg. diabetes, systematic sclerosis, liver neoplasia and HTLV-1 retrovirus adhesion .