ATAD3C is a mitochondrial integral membrane protein and paralog of ATAD3A that arose through gene duplication in primates 1. Although previously considered a pseudogene, ATAD3C is genuinely expressed in human tissues with expression levels similar to ATAD3A 2. ATAD3C functions as a negative regulator of ATAD3A assembly and mitochondrial function. Overexpression of ATAD3C reduces cell proliferation and oxygen consumption while increasing reactive oxygen species production by incorporating into ATAD3A protein complexes and reducing their size 2. This disrupts respiratory chain organization, promoting accumulation of respiratory complex III dimers at the expense of supercomplex assembly 2. ATAD3C is clinically relevant through its involvement in dominant-negative pathological variants. De novo duplications at the ATAD3 locus produce chimeric ATAD3A/ATAD3C fusion proteins causing lethal perinatal cardiomyopathy, hyperlactacidemia, encephalopathy, and corneal clouding 34. These duplications dramatically reduce mitochondrial oxidative phosphorylation complex I activity in cardiac tissue 3. ATAD3 locus duplications now rank among the five most common causes of nuclear-encoded pediatric mitochondrial disease, though diagnosis is frequently missed due to the repetitive nature of this genomic region 3.