Objective: Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a rare disease characterized by the kidney loss of magnesium and calcium and by bilateral medullary nephrocalcinosis. It is caused by mutations in the CLDN16 and CLDN19 genes. In this study, we aimed to present the clinical and laboratory findings of 5 patients with 2 different pathogenic variations. Methods: The clinical features and the detected variants in the CLDN16 gene of 5 children with familial hypomagnesemia with hypercalciuria and nephrocalcinosis from 2 different families are presented. Results: The median age of the 5 female patients included in the study was 11.2 years. The monoamniotic monochorionic twins from the first family had similar clinical and laboratory findings. In these patients, a previously defined pathogenic variant (a homozygous variant of c.710G>A (p.W237*)) in the CLDN16 gene was detected. The 3 sisters from the second family had variable estimated glomerular filtration rates, height and weight severe short stature values, serum calcium, magnesium, uric acid, parathyroid hormone values, as well as variable 24-hour urine calcium, magnesium, citrate, protein, and fractional excretion of magnesium (FEMg2+ %). A novel c.646C>A (p.R216S) homozygous, likely pathogenic variant was detected in the CLDN16 gene of the 3 sisters. Conclusions: Our findings showed that, despite the same mutation, the clinical features of the siblings with familial hypo- magnesemia with hypercalciuria and nephrocalcinosis may differ significantly while monozygotic twins with the same muta- tion had similar phenotypes. This suggests that some other genetic factors are playing a role in the kidney failure process of patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis who have mutations in the CLDN16 gene.