Methylmalonic aciduria and homocystinuria, cobalamin C (cblC) type, is the most frequent inborn error of vitamin B12metabolism. The clinical phenotype includes systemic symptoms and neurological decompensation. Affected patients can be divided into two broad groups, as early-onset and late-onset. We present a Turkish patient who had neurological impairment at the age of four years as presented with late-onset cblC defect. Homozygous c.394C