Heterozygous mutations and deletions of the gene that encodes the transcription factor hepatocyte nuclear factor 1β (HNF1B) are the commonest known monogenic cause of developmental kidney disease. However, diagnosis remains challenging due to phenotypic variability and frequent absence of a family history. There is also no consensus as to when HNF1B genetic testing should be performed. This thesis includes work looking at the identification of HNF1B-associated disease. An HNF1B score was developed in 2014 to help select appropriate patients for genetic testing. The aim in chapter 2 was to test the clinical utility of this score in a large number of referrals for HNF1B genetic testing to the UK diagnostic testing service for the HNF1B gene. An HNF1B score was assigned for 686 referrals using clinical information available at the time of testing; performance of the score was evaluated by receiver-operating characteristic curve analysis. Although the HNF1B score discriminated between patients with and without a mutation/deletion reasonably well, the negative predictive value of 85% reduces its clinical utility. HNF1B-associated disease is due to an approximate 1.3 Mb deletion of chromosome 17q12 in about 50% of individuals. This deletion includes HNF1B plus 14 additional genes and has been linked to an increased risk of neurodevelopmental disorders, such as autism. The aim in chapter 3 was to compare the neurodevelopmental phenotype of patients with either an HNF1B intragenic mutation or 17q12 deletion to determine whether haploinsufficiency of the HNF1B gene is responsible for this aspect of the phenotype. Brief behavioural screening showed high levels of psychopathology and impact in children with a deletion. 8/20 (40%) patients with a deletion had a clinical diagnosis of a neurodevelopmental disorder compared to 0/18 with a mutation, P=0.004. 17q12 deletions were also associated with more autistic traits. Two independent clinical geneticists were able to predict the presence of a deletion with a sensitivity of 83% and specificity of 79% when assessing facial dysmorphic features as a whole. These results demonstrate that the 17q12 deletion but not HNF1B intragenic mutations are associated with neurodevelopmental disorders; we conclude that the HNF1B gene is not involved in the neurodevelopmental phenotype of these patients. Extra-renal phenotypes frequently occur in HNF1B-associated disease, including diabetes mellitus and pancreatic hypoplasia. Faecal elastase-1 levels have only been reported in a small number of individuals, the majority of which have diabetes. In chapter 4 we measured faecal elastase-1 in patients with an HNF1B mutation or deletion regardless of diabetes status and assessed the degree of symptoms associated with pancreatic exocrine deficiency. We found that faecal elastase-1 deficiency is a common feature of HNF1B-associated renal disease even when diabetes is not present and pancreatic exocrine deficiency may be more symptomatic than previously suggested. Faecal elastase-1 should be measured in all patients with a known HNF1B molecular abnormality complaining of chronic abdominal pain, loose stools or unintentional weight loss. Hypomagnesaemia is a common feature of HNF1B-associated disease and is due to renal magnesium wasting. The aim in chapter 5 was to measure both serum and urine magnesium and calcium levels in individuals with an HNF1B molecular defect and compare to a cohort of patients followed up in a general nephrology clinic in order to assess their potential as biomarkers for HNF1B-associated disease. The results of this pilot study show that using a cut-off for serum magnesium of ≤0.75 mmol/L was 100% sensitive and 87.5% specific for the presence of an HNF1B mutation/deletion. All individuals in the HNF1B cohort had hypermagnesuria with fractional excretion of magnesium >4%; a cut-off of ≥4.1% was 100% sensitive and 71% specific. This suggests serum magnesium levels and fractional excretion of magnesium are highly sensitive biomarkers for HNF1B-associated renal disease; if these results are confirmed in a larger study of patients with congenital anomalies of the kidneys or urinary tract they could be implemented as cheap screening tests for HNF1B genetic testing in routine clinical care.