Neurologically normal development of a patient with severe methionine adenosyltransferase I/III deficiency after continuing dietary methionine restriction.

•We report a case with severe MAT I/III deficiency, who had homocystinuria.•We identified novel compound heterozygous mutations of the MAT1A gene.•Plasma methionine levels were kept within 750μmol/L up to 5years old.•Patient's physical growth and psychomotor development have been normal.•Strict methionine restriction may prevent neurological deteriorations.