An Eighteen months old girl  was admitted with delay in the developmental stages at 36 GH she was born, 2700 g, with a respiratory distress for 12 days. He had applied to our outpatient clinic for 11 months because of no significant progress in the developmental stages. He kept his head for four months. Supported seating started 7-8 months, no seating without support. Eye contact is weak, sese orientation.There is a 1st degree cousin marriage between parents. She has two healthy brothers. It was learned that a brother died who lived with similar complaints at the age of two years.There was extensive hypotonia in the physical examination.  Head circumference: measured 41.5 cm (3-10 p). Laboratory: Serum homocysteine:> 50 micromol / L;Urine organic acid test:Methyl malonic acid level is high;Plasma amino acid analysis: Methionine level is low. Monitoring: Clinical, biochemical and brain MR findings were requested for genetic analysis with MTHFR deficiency in the anterior. Betaine has started to support methionine and cobalamin. In the first month of treatment, it was learned that the interest in the neighborhood increased, the mother answered her voice by making voices, and sometimes she smiled. Genetic test confirmed us.