As part of an ongoing search for susceptibility loci for NIDDM, we tested 19 genes whose products are implicated in insulin secretion or action for linkage with NIDDM. Loci included the G-protein–coupled inwardly rectifying potassium channels expressed in β-cells (KCNJ3 and KCNJ7), glucagon (GCG), glucokinase regulatory protein (GCKR), glucagon-like peptide I receptor (GLP1R), LIM/homeodomain islet-1 (ISL1), caudal-type homeodomain 3 (CDX3), proprotein convertase 2 (PCSK2), cholecystokinin B receptor (CCKBR}, hexokinase 1 (HK1), hexokinase 2 (HK2), mitochondrial FAD-glycerophosphate dehydrogenase (GPD2), liver and muscle forms of pyruvate kinase (PKL, PKM), fatty acid–binding protein 2 (FABP2), hepatic phosphofructokinase (PFKL), protein serine/threonine phosphatase 1 beta (PPP1CB), and lowdensity lipoprotein receptor (LDLR). Additionally, we tested the histidine-rich calcium locus (HRC) on chromosome 19q. All regions were tested for linkage with microsatellite markers in 751 individuals from 172 families with at least two patients with overt NIDDM (according to World Health Organization criteria) in the sibship, using nonparametric methods. These 172 families comprise 352 possible affected sib pairs with overt NIDDM or 621 possible affected sib pairs defined as having a fasting plasma glucose value of >6.1 mmol/1 or a glucose value of >7.8 mmol/1 2 h after oral glucose load. No evidence for linkage was found with any of the 19 candidate genes and NIDDM in our population by nonparametric methods, suggesting that those genes are not major contributors to the pathogenesis of NIDDM. However, some evidence for suggestive linkage was found between a more severe form of NIDDM, defined as overt NIDDM diagnosed before 45 years of age, and the CCKBR locus (11pl5.4; P = 0.004). Analyses of six additional markers spanning 27 cM on chromosome l ip confirmed the suggestive linkage in this region. Whether an NIDDM susceptibility gene lies on chromosome l ip in our population must be determined by further analyses.