Cohen syndrome is a rare autosomal-recessive disorder characterized by intellectual disability, myopia, hypotonia, and skeletal malformations. Its clinical diagnosis is impeded by marked inter- and intrafamilial phenotypic variability. Gene VPS13B that carries disease-associated mutations has 62 exons, making Sanger sequencing of the entire gene unsuitable for routine clinical use due to high costs. In this work we report a case of Cohen syndrome in a brother and sister born to a mixed Abazin-Circassian marriage and diagnosed with moderate mental retardation. Both patients had psychomotor retardation, were unable to study at school, and never learned to read, write and count. Although the patients shared a few nonspecific phenotypic characteristics, phenotypic differences made it impossible to arrive at a clear diagnosis. Therefore, whole exome sequencing was performed revealing the single nucleotide variant с.7603C>T that results in the premature stop codon R2535* in VPS13B. This mutation was found in the mother, the affected sibs and one of the two other healthy sibs. The second mutation remained undetected. Considering the identified mutation and the analyzed phenotypic traits, we concluded Cohen syndrome in both patients.