MMA is classified into two main forms according to phenotype, including isolated methylmalonic acidurias and combined methylmalonic aciduria and homocystinuria, and caused by the defects of 10 genes, including MUT, MMAA, MMAB, CD320, MMADHC, LMBRD1, HCFC1, ABCD4, MCEE, and SUCLA2. According to previous reports, the incidence of MMA was 1/50,000 in Japan (Shigematsu et al., 2002), 1/85,000 in Taiwan of China (Cheng et al., 2010), and 1/250,000 in Germany (Schulze et al., 2003). The gene discussed is SUCLA2; the disease is homocystinuria.