Variant KHK (ketohexokinase) protein fails to catalyze the phosphorylation of fructose to yield fructose 1-phosphate (Fru 1-P), the first step of fructose catabolism in the liver. This defect is associated with essential fructosuria, a rare benign condition characterized by elevated urinary fructose levels associated with consumption of fructose. Two missense mutant alleles have been identified in DNA sequencing studies of affected individuals (Bouthron et al. 1994). One, G40R, has no detectable activity. The second, A43T, encodes a protein whose liver ("A") isoform is inactive but whose peripheral ("C") isoform, though thermally unstable, retains some activity (Asipu et al. 2003).