MMACHC and methylmalonic acidemia: Among combined methylmalonic acidemia cases in China, MMACHC gene mutation is the most common, accounting for approximately 99% of cases, and the most common mutant subtype is c.609G>A (p.W203X), followed by c.80A>G (p.Q27R) and c.658-660delAAG (p.K220del) (Liu et al., 2010; Yu et al., 2021).