To date, one patient with infant onset ECHS1D has been reported to be a compound heterozygote that possesses the F33S variant on one allele and the N59S variant on the other allele.6 ECHS1 expression was significantly reduced in that patient’s fibroblasts, similar to what we observed in our mice, although it is unknown how the N59S variant impacted ECHS1 expression. This evidence concerns the gene ECHS1 and mitochondrial short-chain Enoyl-Coa hydratase 1 deficiency.