Sulfatases are hydrolytic enzymes which cleave sulfate ester bonds of their substrates . Sulfatases show highly conserved sequence features, especially at the N-terminal region where the catalytic site is located. Arylsulfatase K (ASK) is a water-soluble enzyme found in the endoplasmic retuculum (ER) . Sequence alignment studies show that the catalytic site of STS shares many key residues within ASK in addition to the above mentioned universally conserved residues, suggesting that the function and substrates of STS may be similar to that of ASK. Based on evolutionary analysis a cysteine residue, which is present in all sulfatases are highly conserved across the evolution of the sulfatase family. With exceptions in some bacteria including ASK of Bacteroides thetaiotaomicron, which shows a serine residue in place of cysteine.