This domain is found in squalene epoxidase (SE) and related proteins which are found in taxonomically diverse groups of eukaryotes and also in bacteria. SE was first cloned from Saccharomyces cerevisiae where it was named ERG1. It contains a putative FAD binding site and is a key enzyme in the sterol biosynthetic pathway. Putative transmembrane regions are found to the protein's C-terminus.