Mitochondrial small peptide that orchestrates a two-pronged immunoregulatory mechanism in response to inflammatory stimuli. Replaces the canonical cytochrome c oxidase subunit COXFA4 in mitochondrial complex IV and targets it to degradation, attenuating mitochondrial respiration, reducing membrane potential as well as ROS generation and modulates innate immune signaling (PubMed:33837217, PubMed:34878835). Additionally, initiates stress-independent autophagy by lowering ATP levels, activating AMPK-ULK1 signaling, boosting glutathione production, and thereby mitigating oxidative stress and supporting immune tolerance (PubMed:38296961). {ECO:0000269|PubMed:33837217, ECO:0000269|PubMed:34878835, ECO:0000269|PubMed:38296961}. This is the function of Ensembl gene identifier ENSG00000166920 (COXFA4L3, cytochrome c oxidase associated subunit FA4L3).