The RN(-) (rendement Napole, French for Napole yield) phenotype is common in Hampshire pigs and is characterized by a 70% increase in glycogen content in skeletal muscle and large effects on meat characteristics (pH, water content, technological yield and lean meat content). The phenotype is controlled by an autosomal dominant allele designated RN (-). The protein kinase AMP-activated gamma 3 subunit gene, PRKAG3, which encodes the gamma 3 isoform of AMP-activated protein kinase (AMPK), was identified as the causative gene for this phenotype by a pure positional cloning approach. There are now several lines of evidence supporting our interpretation that the RN(-) phenotype is caused by a missense mutation (Arg(200)-->Gln) in PRKAG3. Recent data from another group have revealed the presence of a third functional allele at the PRKAG3 locus, probably caused by a Val(199)-->Ile missense mutation. This allele has opposite effects compared with RN, as it is associated with a low glycogen content. We have confirmed the phenotypic effect of this third allele in a meat-quality study of a Hampshire/Landrace intercross. A physiological characterization of RN(-) carriers and normal pigs showed that the RN(-) pigs utilized glycogen during exercise to the same extent as normal pigs and they showed a significantly faster resynthesis of glycogen after exercise. The results strongly suggest that the Arg(200)-->Gln substitution is not associated with a defect in glycogen degradation, but rather with an increased glucose uptake in skeletal muscle.