Methylmalonic acidemia with homocystinuria due to CblC deficiency (OMIM# 277400), the most common intracellular disorder of vitamin B12 metabolism, is an autosomal recessive disease caused by a mutation in the MMACHC gene (OMIM* 609831) [1] or by a specific splice-site variant in the neighboring PRDX1 (OMIM* 176763) associated with an epigenetic form of CblC (epi-CblC) [2]. Here, MMACHC is linked to methylmalonic aciduria and homocystinuria type cblC.