Two siblings with vitamin B6-nonresponsive cystathionine β-synthase deficiency and differing blood methionine levels during the neonatal period

We present two siblings with vitamin B6-nonresponsive homocystinuria due to a deficiency of cystathionine β-synthase who had different levels of methionine in the blood during the neonatal period, even though they had the same genetic defect. One of them was missed in the screening of newborns for homocystinuria. Special care should be taken in screening neonates for homocystinuria using the blood level of methionine.