Methylmalonic academia (MMA) is a rare inborn error of the metabolism due to defects of the methylmalonyl-CoA mutase (MUT) enzyme that result in methylmalonic acid accumulation in the blood and increased excretion in urine. Affected patients can manifest vomiting, dehydration, hypotonia, developmental delay and failure to thrive. The emergency treatment of the newborns with MMA mainly comprises rehydration and promotion of anabolism, followed by long-term dietary management that mostly includes restriction of propiogenic amino acids by a low protein diet and carnitine and vitamin B12 supplementation. The effects of MUT deficiency on the cellular damage in patients is still unknown. Fibroblasts obtained from two distinct MMA patients were analyzed by shotgun label-free quantitative (LFQ) proteomics.