Abstract Very long chain fatty acids (VLC-FA, ≥ C28) are synthesized by the Elongation of Very Long Chain Fatty Acids-4 (ELOVL4) enzyme and are essential for life such that depletion of tissue VLC-FA (due to knockout of ELOVL4) causes neonatal lethality. The ELOVL4 protein is expressed in the retina, Meibomian glands, brain, skin, and testes. In each of these tissues, the enzyme makes two different classes of VLC-FA. One class is VLC-saturated fatty acids (VLC-SFA) that are found mainly in the skin, brain, Meibomian glands and tear film. The other is VLC-polyunsaturated fatty acids (VLC-PUFA) that are found in the retina, testes and sperm. Since VLC-FA are essential for the normal function of these tissues, mutations in ELOVL4 cause distinct tissue- specific disorders like Stargardt-like macular dystrophy (STGD3), spinocerebellar ataxia-34 (SCA34), skin pathologies, seizures and death. However, the mechanisms by which one tissue makes mainly VLC-SFA and the other VLC-PUFA, and how the VLC-FA exert their importance in each tissue remain unknown. We seek to understand the molecular basis of this phenomenon with the goal of developing therapeutic targets for attenuating disease progression. Our lab and others have shown that conditional deletion of retinal Elovl4 or expression of the mutant Elovl4 leads to depletion of VLC-SFA and VLC-PUFA, which affect retinal structure and function. We also showed that the STGD3 mutant ELOVL4 lacks VLC-PUFA biosynthesis and exerts a dominant negative effect on wild type ELOVL4 (WT ELOVL4) in vitro and in vivo causing decreased VLC-FA biosynthesis. These suggest that VLC-FA are necessary for maintenance of photoreceptor health and function. To understand how one set of mutations causes vision loss and another causes ataxia, we successfully generated a Long Evans (LE) rat knock-in model of the SCA34 ELOVL4 mutation (c.736T>G) that recapitulates the human disorder. Consequently, we now have animal models of STGD3 and SCA34. We hypothesize that tissue-specific factors determine which type of fatty acid is made in specific tissues and that the pathological differences found in patients with STGD3 and SCA34 result from differences in the types of VLC-FAs produced directly by the mutant enzymes or from effects of the mutant enzymes on VLC-FA produced by the wild type ELOVL4 enzyme. We propose two specific aims: 1) To determine how different ELOVL4-expressing tissues control the VLC-SFA and VLC-PUFA biosynthetic activity of ELOVL4. 2) To determine how the different ELOVL4 mutations affect the quality and quantity of the VLC-FA that they synthesize. The results would provide better mechanistic insight into the relative contributions of the depletion of VLC-FA and the mutant ELOVL4 in the progression of the mutant ELOVL4 disorders. This could potentially provide an immediate positive impact by paving the way for development of potential therapeutic approaches to rescue these disorders.