Abstract. The problem of early diagnosis and treatment of orphan (rare) diseases is unresolved in the world medical community. This is mainly due to the fact that some of this cohort of diseases have an incidence of less than 1:1,000,000. In most cases, the diagnosis is made after the height of the disease and quite often in the presence of irreversible changes. Department of Psychoneurology, State Institution «Institute of Pediatrics, Obstetrics and Gynecology named after acad. O. M. Lukyanova of the NAMS of Ukraine» has been engaged in the diagnosis and treatment of orphan diseases since 2012 and has experience in the diagnosis and treatment of orphan diseases, 80 % of which are manifested by neurological symptoms. A large number of orphan diseases in children occur under the clinical «masks» of diseases such as cerebral palsy, hydrocephalus, epilepsy, mental retardation and autism spectrum disorders. It is important that the pediatrician be vigilant about orphan diseases and approach the diagnosis from the etiology and pathogenesis of development, rather than clinical manifestations. In recent years, doctors have armed themselves with modern diagnostic techniques such as MRI, MR spectroscopy, next generation sequencing (NGS), whole exome sequencing (WES), and whole genome sequencing (WGS). But the question remains about the interpretation of the results obtained, the algorithm for early diagnosis and the patient's clinical route. In this publication, we would like to share our experience in the diagnosis and treatment of orphan diseases, taking into account modern protocols and data based on our own observations and international scientific sources. We give an example of a rare (orphan) disease caused by a POLG mutation and describe the entire diagnostic process from the appearance of the first symptoms of the disease to the establishment of the final diagnosis Keywords: orphan diseases, perinatal neurology, mitochondrial diseases, leukodystrophies, complete exome sequencing