Methylcobalamin deficiency cbl G type is a rare condition that occurs when the body is unable to process certain amino acids (building blocks of protein) properly. In most cases, signs and symptoms develop during the first year of life; however, the age of onset can range from infancy to adulthood. Common features of the condition include feeding difficulties, lethargy, seizures, poor muscle tone (hypotonia), developmental delay, microcephaly (unusually small head size), and megaloblastic anemia. Methylcobalamin deficiency cbl G type is caused by changes (mutations) in the MTR gene and is inherited in an autosomal recessive manner. Treatment generally includes regular doses of hydroxycobalamin (vitamin B12). Some affected people may also require supplementation with folates and betaine. The disease is methylcobalamin deficiency type cblG (MONDO_0009609). Also known as: functional methionine synthase deficiency type cblG.