The present invention relates to an isolated DNA molecule encoding a fagopyritol synthase. A method for producing a fagopyritol, an insulin mediator, an insulin mediator analog, an insulin mediator homolog, or an insulin mediator inhibitor is also described. The method includes providing a fagopyritol synthase, providing a substrate comprising a galactosyl donor and a galactosyl acceptor, and combining the fagopyritol synthase with the substrate under conditions effective produce a fagopyritol, an insulin mediator, an insulin mediator analog, an insulin mediator homolog, or an insulin mediator inhibitor.