ECSIT (Evolutionary Conserved Signaling Intermediate in Toll pathways) is a multifunctional adaptor protein with dual roles in innate immunity and mitochondrial homeostasis. As a component of the MCIA (Mitochondrial Complex I Assembly) complex, ECSIT facilitates mitochondrial respiratory chain complex I assembly 1. ECSIT binding induces conformational changes in ACAD9, converting it from a fatty acid oxidation enzyme to a complex I assembly factor 1. In innate immune signaling, ECSIT forms a TAK1-ECSIT-TRAF6 complex essential for TLR4-mediated NF-κB activation and inflammatory cytokine production 2. ECSIT undergoes ubiquitination by TRAF6 during TLR4 signaling, a process suppressible by PRDX1 3. Beyond inflammation, ECSIT regulates memory CD8+ T cell differentiation by controlling fumarate synthesis and TCF-1 expression, with elevated ECSIT correlating with improved cancer patient survival 4. ECSIT also maintains mitochondrial function through regulation of reactive oxygen species production and membrane potential 5. A novel mitochondrial-localized ECSIT-X4 isoform protects against pressure overload-induced cardiac hypertrophy by interacting with mitochondrial STAT3 6. Dysregulation of ECSIT contributes to multiple pathologies including Alzheimer's disease, cardiac hypertrophy, and cancer, establishing it as both an immune regulator and metabolic tumor suppressor 7.