The etiology of pectus excavatum is not fully clear yet, but it has been found that the incidence of pectus excavatum greatly increases among patients with connective tissue diseases, possibly related to the disruption of the balance between growth genes and inhibitory genes affecting the cartilage on both sides of pectus excavatum. Moreover, it is also found that the complication of pectus excavatum significantly increases among patients with Marfan syndrome (also a type of connective tissue disease) and Noonan syndrome. In children with congenital airway stenosis and bronchopulmonary dysplasia, the incidence of pectus excavatum also significantly increases. This suggests that the causes of pectus excavatum are directly or indirectly related to genetics and heredity, and regardless, the causes of pectus excavatum, both acquired and congenital, are directly related to genes and heredity.