Hexokinase II (HKII) plays a central role in the intracellular metabolism of glucose in skeletal muscle, catalysing the phosphorylation of glucose to glucose 6-phosphate. It is therefore considered to be a potentially important candidate gene in the development of insulin resistance and non-insulin-dependent diabetes mellitus (NIDDM). The aim of this study was to screen the HKII gene for mutations in NIDDM subjects from insulin-resistant families. Insulin sensitivity was assessed in unaffected first degree relatives from families with two or more living NIDDM subjects, and 15 families were identified as being insulin resistant. In 15 NIDDM subjects (one from each family) and 4 normoglycaemic control subjects, all 18 exons of the HKII gene were amplified by the polymerase chain reaction, and the products screened for mutations using a combination of single-stranded conformational polymorphism analysis and direct sequencing. Six sequence variations were detected in the NIDDM subjects; four silent polymorphisms [GAT vs GAC at codon 251 in exon 7, AAT vs AAC at codon 692 in exon 15, CCG vs CCC at codon 736 in exon 15, and CTG vs CTA at codon 766 in exon 16]; a single base change [T-+C], 22 base pairs distal to the exon-intron junction of exon 17 in the 5 '-splice donor; and a single amino acid substitution [Gln142--+His] in exon 4, which was identified in 6 of the 15 NIDDM subjects. The frequency of the mutated codon 142 allele however, was comparable between NIDDM subjects with familial NIDDM (n=56) and normoglycaemic control subjects (n = 48) (18.8 % and 14.6 % for NIDDM subjects and control subjects respectively; g 2 = 0.6, p > 0.25). In addition, measures of insulin sensitivity were comparable in normal glucose tolerant subjects with (n = 20) and without (n --40) the codon 142 polymorphism. In conclusion: (1) mutations in the coding regions of the HKII gene are unlikely to be major determinants in the development of insulin resistance and familial NIDDM; although (2) the influence of the codon 142 mutation in combination with other abnormalities of the insulin-signalling pathway on insulin action remain to be addressed. [Diabetologia (1996) 39: 322-328] Keywords Hexokinase II, candidate gene analysis, insulin resistance, familial non-insulin-dependent diabetes mellitus.