ATP5MJ (ATP synthase membrane subunit j) is a minor structural component of mitochondrial ATP synthase (Complex V), the enzyme complex responsible for ATP synthesis coupled to the proton gradient generated by the electron transport chain 1. As part of the F(0) membrane domain, ATP5MJ participates in the proton-translocating channel essential for oxidative phosphorylation 1. ATP5MJ is required to maintain adequate ATP synthase population levels in mitochondria; its depletion correlates with reduced ATP synthesis capacity 2. Clinically, ATP5MJ dysfunction has been implicated in cardiac pathology. In ischemic cardiomyopathy (ICM) patients, ATP5MJ expression is significantly upregulated (fold change = 1.33, p < 0.05) and correlates with established markers of cardiac remodeling and ventricular dysfunction, including left ventricular end-systolic/end-diastolic diameters and ejection fraction 3. This dysregulation occurs within a broader pattern of Complex V overexpression in heart failure, suggesting mitochondrial bioenergetic abnormalities contribute to disease pathogenesis. Recent evidence indicates ATP5MJ contains a selenocysteine insertion sequence (SECIS) element in its 3' UTR, enabling selenocysteine incorporation through stop codon readthrough 4. ATP5MJ has also been identified as a transcriptomic biomarker for predicting chemotherapy response in triple-negative breast cancer 5, suggesting broader roles in cellular stress responses.