Maturity onset diabetes is a genetic form of diabetes mellitus characterized by an early age at onset and several etiologic genes for this form of diabetes have been identified in many patients. Maturity onset diabetes type 2 [MODY2 (#125851)] caused by mutations in the glucokinase gene (GCK). Although its prevalence is not clear, it is estimated that 1%–2% of patients with diabetes have the monogenic form. The aim of this study was to evaluate the molecular spectrum of GCK gene mutations in 177 Turkish MODY type 2 patients. Mutations in the GCK gene were identified in 79 out of 177. All mutant alleles were identified, including 45 different GCK mutations, 20 of which were novel.