ECI1 (enoyl-CoA delta isomerase 1) is a mitochondrial enzyme catalyzing a critical step in the β-oxidation of unsaturated fatty acids 1. The enzyme isomerizes both 3-cis and 3-trans double bonds into the 2-trans form in enoyl-CoA species, with preference for 3Z-enoyl-CoAs, facilitating degradation of mono- and polyunsaturated fatty acids 1. ECI1 exhibits functional redundancy with ECI2; mice deficient in ECI1 alone display mild phenotypes with elevated unsaturated acylcarnitines, whereas dual ECI1/ECI2 knockdown causes pronounced metabolic disruption 2. In cancer biology, ECI1 drives aggressive phenotypes: overexpression increases prostate cancer cell growth, motility, and metastatic potential, with ECI1 expression serving as a strong independent predictor of biochemical recurrence (HR: 3.65) and poor overall survival 3. ECI1 supports metastatic progression by enabling cancer cells to process polyunsaturated fatty acids for β-oxidation, creating targetable metabolic dependencies 4. ECI1 expression is dysregulated in premature ovarian insufficiency and diabetic cardiomyopathy, correlating with impaired mitochondrial function and altered immune responses 56. Additionally, ECI1 has been identified as a novel plasma protein associated with glaucoma risk 7. These findings establish ECI1 as both a fundamental metabolic enzyme and an emerging therapeutic target across multiple disease contexts.