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Aims: HNF1B syndrome is caused by defects in the hepatocyte nuclear factor 1B (HNF1B) gene, which leads to maturity‐onset diabetes of the young type 5 and congenital organ malformations. This study aimed to identify a gene defect in a patient presenting with diabetes and severe diarrhea, while also analyzing the prevalence of hypomagnesemia and its correlation with the HNF1B genotype. Materials and Methods: Whole exome sequencing was used to identify responsible point mutations and small indels in the proband and their family members. Multiplex ligation‐dependent probe amplification was carried out to identify HNF1B deletions. Furthermore, an analysis of published data on 539 cumulative HNF1B cases, from 29 literature sources, was carried out to determine the correlation between the HNF1B genotype and the phenotype of serum magnesium status. Results: Using multiplex ligation‐dependent probe amplification, we identified a de novo heterozygous HNF1B deletion in the patient, who showed dorsal pancreas agenesis and multiple kidney cysts, as detected by magnetic resonance imaging. Magnesium supplementation effectively alleviated the symptoms of diarrhea. Hypomagnesemia was highly prevalent in 192 out of 354 (54.2%) patients with HNF1B syndrome. Compared with patients with intragenic mutations, those with HNF1B deletions were more likely to suffer from hypomagnesemia, with an odds ratio of 3.1 (95% confidence interval 1.8–5.4). Conclusions: Hypomagnesemia is highly prevalent in individuals with HNF1B syndrome, and those with HNF1B deletion are more susceptible to developing hypomagnesemia compared with those with intragenic mutations. The genotype–phenotype associations in HNF1B syndrome have significant implications for endocrinologists in terms of genotype detection, treatment decisions and prognosis assessment. [ABSTRACT FROM AUTHOR]

HighlightsThis study reports a novel frameshift mutation c.212dupG (p.Gly73fs) in the insulin (INS) gene causing maturity onset diabetes of the young (MODY) 10, one of the rare types of MODY, identified in seven family members.Screening for mutations in identified MODY genes is warranted in patients who require insulin, are negative for autoantibodies but have a family history of diabetes. [ABSTRACT FROM AUTHOR]