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Background: Biallelic QRSL1 mutations cause mitochondrial 'combined oxidative phosphorylation deficiency-40' (COXPD40). COXPD40 has been reported to be invariably lethal in infancy. Adrenal insufficiency was weakly reported and investigated among seven previously reported patients with COXPD40. Objective: We report the clinical, biochemical, molecular, and functional characteristics of a patient with adrenal insufficiency due to C0XPD40. Methods: The medical history and adrenal function tests were examined. Genetic analysis was performed using whole-exome sequencing. Mitochondrial function was tested using mitochondrial membrane potential (MMP) and superoxide dismutase (SOD) enzyme assays. Results: An 8-year-old boy was investigated for adrenal insufficiency. He also had mild developmental delay, sensorineural hearing loss, hypertrophic cardiomyopathy, nephrocalcinosis, elevated parathyroid hormone and creatine kinase, and lactic acidosis. Biallelic novel QRSL1 variants (c.300T>A;Y100* and c.610G>A;G204R) were identified. Oxidative damage in mitochondria was shown by reduced MMP and SOD assays in the patient compared to controls (P< 0.0001). Adrenal function tests revealed a 'primary adrenal insufficiency other than congenital adrenal hyperplasia' (non-CAH PAI) with an isolated glucocorticoid deficiency. In the 8-year follow-up, having the longest survival of reported COXPD40 patients, he had preserved mineralocorticoid functions and gonadal steroidogenesis. Conclusion: Biallelic QRSL1 mutations can cause non-CAH PAI. Adrenal functions should be monitored in mitochondrial disorders to improve clinical outcomes. [ABSTRACT FROM AUTHOR]