Methylmalonic academia is the most common inherited organic acid metabolic disease in China, which is usually considered to have a poor prognosis. We recruited a large cohort of 365 mut-type MMA patients, investigated their phenotype and genotype. Participation in tandem mass spectrometry expanded newborn screening, vitamin B12 responsive and late onset are favorable factors for the prognosis. Patients carrying the c.1663G>A variation had a milder clinical phenotype. MS/MS expanded NBS facilitates early diagnosis and treatment for the disease, which should be promoted further. (By Drs. Lili Liang and Lianshu Han, https://jmg.bmj.com/content/early/2023/06/13/jmg-2022-108682 )