Human Sulfatase Modifying Factor 1 (SUMF1) is a 42kDa protein. SUMF1 is a Ca2+-binging member of the sulfatase-modifying factor family. SUMF1 is a soluble ER lumenal glycoprotein, it converts inactive sulfatases into an active form by transforming a catalytic site cysteine into a formylglycine residue. In the ER, SUMF1 can exist as either a monomer, or a disulfide-linked homodimer or a heterodimer with SUMF2. Three splice isoforms are known.