MTHFR and homocystinuria: Examination of plasma Hcy concentrations in patients diagnosed with genetic homocystinurias, namely, methylentetrahydrofolate reductase (MTHFR) deficiency, and methylmalonic aciduria combined with homocystinuria type C (MMACHC, cblC) resulted in the expected elevations compared to the healthy control group (Figure 3b).