The ETHE1 c.487C>T mutation is a missense mutation at the 163 amino acid position where one other pathogenic mutation has been reported (c.488G>A) indicating that this residue is critical to the function of the protein (PM5_moderate). There are multiple lines of computational evidence supporting a deleterious effect (PP3_supporting). Functional studies performed in ETHE1 isolated from E-coli cells demonstrated 10 fold reduced ETHE1 catalytic activity when compared to controls (PS3_supporting; PMID:25198162). The c.487C>T mutation has been reported in four unrelated affected individuals including three who were homozygous for the mutation (PMID:16183799; PMID:16828325; PMID:28698729) and one proband where the c. 487C>T was found in trans with another reportedly pathogenic ETHE1 mutation (PMID:18593870) (PM3_strong). One of these patients demonstrated a clinical phenotype specific to ethylmalonic encephalopathy including chronic diarrhea, petechiae, acrocyanosis, and developmental delay in addition to the biochemical findings urinary ethylmalonic acid and elevated blood C4-acylcarnitine esters and blood C5-acylcarnitines (PP4_moderate; PMID: 16828325). A larger family including 7 children has been reported where homozygous c.487C>T mutation was identified in the three affected siblings and segregated with disease in the unaffected siblings (PP1_strong; PMID:14732903). In summary, this variant meets criteria to be classified as pathogenic for ethylmalonic encephalopathy in an autosomal recessive manner. ETHE1-specific ACMG/AMP criteria applied PM5_moderate; PP3_supporting; PS3_supporting; PM3_strong; PP4_moderate; PP1_strong.