Costello syndrome is an extremely rare disorder that affects multiple organ systems of the body. This condition is characterized by growth delays after birth; short stature; extra loose skin on the neck, palms of the hands, fingers, and soles of the feet; noncancerous tumors (papillomata) around the face and anus; developmental delay and intellectual disability; and a characteristic facial appearance. Other physical features may include the development of dry hardened skin on the palms of the hands and the soles of the feet (palmoplantar hyperkeratosis), abnormally deep creases on the palms and soles, and/or abnormally flexible joints of the fingers (hyperextensible). There is an increased incidence of congenital abnormalities of the heart and thickening of the heart muscle called a cardiomyopathy. Characteristic craniofacial features may include an abnormally large head (macrocephaly); course facial features; unusually thick lips; and/or abnormally wide nostrils (nares). Affected individuals have an increased lifetime risk to develop specific malignant tumors. Costello syndrome is an autosomal dominant genetic condition caused by mutations in the HRAS gene.