One of the causes of sudden death may be complications that develop as a result of connective tissue dysplasia (CTD). CTD is a genetically determined condition caused by defects in fibrous structures and the ground substance of connective tissue and leading to a violation of the shape of organs and systems. There are hereditary connective tissue disorders (HCTD) (monogenic CTD) and undifferentiated (polygenic) CTD. Differential diagnosis between HCTD and polygenic CTD is difficult even for specialists, while the prognosis of patients is different. This article describes the clinical case of a 41-year-old patient with symptoms of HCTD who came to the National Medical Research Center for Preventive Medicine for examination. Molecular genetic testing by exomic sequencing found a new pathogenic variant of the FBN1 gene (FBN1: NM_000138: exon40: c.4932_4935del: p.R1644fs) in a patient and established a diagnosis of Marfan syndrome (MS). Using cascading screening, including genetic screening, during which the parents and son of the proband were examined, MS was diagnosed in the child of proband at the age of 3 years, before the development of severe clinical manifestations of the disease. The use of molecular genetic diagnosis methods makes it possible to diagnose MS as early as possible, before the development of serious complications, and, accordingly, to develop a plan for personalized prevention and treatment.