Differential diagnosis of delayed mental and motor development in children is very wide and demanding. The causes of psychomotor retardation may be perinatal asphyxia resulting in hypoxic encephalopathy, endocrine disorders, disorders nutrition, hereditary metabolic disorders or degenerative CNS disorders and last but not least many also have a number of genetic syndromes. Differential diagnosis of genetic syndromes is often very difficult due to the fact that many diseases with intellectual disabilities do not always have a specific phenotype and are essential for an accurate diagnosis genetic laboratory testing. Due to the unexplained cause of psychomotor retardation and already exhausted diagnostic possibilities, our patient was included to a study to perform whole exome sequencing. Only thanks to this complex genetic analysis was it possible to find a causal one mutation in the PPP2R5D gene (PPP2R5D-related intellectual disability and neurodevelopmental delay, MIM 601646) and make a diagnosis. Currently, approximately 100 patients with this disease are described worldwide. In our case study we present the first patient with this diagnosis in the Czech Republic..