The regulatory G-subunit of the glycogen-associated form of protein phosphatase 1 (PP1) plays a crucial part in muscle tissue glycogen synthesis and breakdown. As impaired insulin stimulated glycogen synthesis in peripheral tissues is considered to be a pathogenic factor in subsets of non-insulin-dependent diabetes mellitus (NIDDM) and obesity, the G-subunit of PP1 should be viewed as a candidate gene for inherited insulin resistance. When applying heteroduplex formation analysis and nucleotide sequencing of PP1G-subunit cDNA from 30 insulin resistant white NIDDM patients two cases were identified as heterozygous carriers of an Asp⁹⁰⁵→Tyr substitution. The carrier prevalence of the PP1G-subunit variant was 18% in 150 healthy subjects and 13% in 313 NIDDM subjects (x² = 1.94, p = 0.16). Twenty-seven healthy subjects volunteered for a 4 h euglycaemic, hyperinsulinaemic clamp in combination with indirect calorimetry in order to elucidate the potential impact of the Tyr⁹⁰⁵ substitution on the whole body glucose metabolism. Interestingly, the Tyr⁹⁰⁵ variant was associated with altered routing of glucose: a decreased insulin stimulated non-oxidative glucose metabolism of peripheral tissues (glycogen synthesis) (p<0.04) and an increased basal glucose oxidation rate (p<0.04) when compared with wild type carriers. A population-based sample of 380 unrelated young healthy Caucasians was examined during a combined intravenous glucose and tolbutamide test to address whether the Asp⁹⁰⁵/Tyr⁹⁰⁵ polymorphism was associated with alterations in insulin secretion which might be secondary to the insulin resistance of skeletal muscle. First phase insulin secretion was unaffected in lean carriers of the codon 905 polymorphism whereas obese carriers of the variant PP1G allele had a 42% (p<0.003) higher first phase insulin response when compared with matched obese wild-type carriers. In conclusion: (i) a widespread PP1G-subunit Asp⁹⁰⁵ /Tyr⁹⁰⁵ polymorphism offers the first major contribution to explain the genetics behind the large variation in insulin-stimulated muscle glycogen synthesis in the general Caucasian population; (ii) the PP1G variant and obesity interact to elicit a postglucose hypersecretion of insulin; and (iii) the findings substantiate the concept that a single genetically determined tissue-specific (skeletal muscle) and pathway-specific (glycogen synthesis) insulin resistance is not by itself sufficient to increase the risk of developing NIDDM.