This domain is found in eukaryotic proteins required for post-translational sulfatase modification (SUMF1). These proteins are associated with the rare disorder multiple sulfatase deficiency (MSD). The protein product of the SUMF1 gene is FGE, formylglycine (FGly),-generating enzyme, which is a sulfatase. Sulfatases are enzymes essential for degradation and remodelling of sulfate esters, and formylglycine (FGly), the key catalytic in the active site, is unique to sulfatases. FGE is localized to the endoplasmic reticulum (ER) and interacts with and modifies the unfolded form of newly synthesized sulfatases. FGE is a single-domain monomer with a surprising paucity of secondary structure that adopts a unique fold which is stabilized by two Ca2+ ions. The effect of all mutations found in MSD patients is explained by the FGE structure, providing a molecular basis for MSD. A redox-active disulfide bond is present in the active site of FGE. An oxidized cysteine residue, possibly cysteine sulfenic acid, has been detected that may allow formulation of a structure-based mechanism for FGly formation from cysteine residues in all sulfatases. In Mycobacteria and Treponema denticola this enzyme functions as an iron(II)-dependent oxidoreductase.