MONDO_0100255 (adenosine kinase deficiency) is a rare inborn error of metabolism characterized by persistent hypermethioninemia with increased levels of S-adenosylmethionine and S-adenosylhomocysteine which manifests with encephalopathy, severe global developmental delay, mild to severe liver dysfunction, hypotonia and facial dysmorphism (most significant is frontal bossing, macrocephaly, hypertelorism and depressed nasal bridge). Epileptic seizures, hypoglycemia and/or cardiac defects (pulmonary stenosis, atrial and/or ventricular septal defect, coarctation of the aorta) may be associated. Clinical picture may range from neurological symptoms only to multi-organ involvement. Also known as: ADK deficiency, ADK hypermethioninemia, MRT8, autosomal recessive intellectual disability 8, autosomal recessive mental retardation 8, hypermethioninemia due to adenosine kinase deficiency, hypermethioninemia encephalopathy due to ADK deficiency, hypermethioninemia encephalopathy due to adenosine kinase deficiency.