Glycerol kinase (GK) is an enzyme at the interface of carbohydrate and fat metabolism. Mutations in the GK gene result in a rare inborn error of metabolism, GK deficiency (GKD), and at least one of these mutations (N288D) is associated with insulin resistance and diabetes mellitus. In an attempt to identify potential modifiers of the GKD phenotype, and to elucidate better the relationship between GKD and diabetes mellitus, we investigated the GK promoter. We examined the GK promoter using in silico methods, transient transfections of GK promoter-luciferase constructs in HepG2 hepatocellular carcinoma cells, and gel shift assays using liver nuclear extracts. We determined that the first 100 bp of the GK 5(') upstream region was sufficient for basal levels of transcription and that there was a functional HNF-4alpha binding site in the first 500 bp of the 5(') upstream region that was important for increased levels of GK expression in vitro. The involvement of both GK and HNF-4alpha in the etiology of diabetes mellitus is intriguing, and we speculate that HNF-4alpha represents a potential modifier of the GKD phenotype.