Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) and trifunctional protein (TFP) deficiency are two fatty acid beta-oxidation disorders, caused by biallelic pathogenic variants of the genes HADHA and/or HADHB, molecularly differentiated but clinically indistinguishable. This evidence concerns the gene HADHB and mitochondrial trifunctional protein deficiency.