NDUFB6 and Leigh syndrome: Both in silico pathogenicity predictions and oxidative phosphorylation (OXPHOS) functional findings in patient's skin fibroblasts (delayed cell growth, isolated CI enzyme defect, decreased basal and maximal oxygen consumption and as well as ATP production, together with markedly diminished levels of the NDUFA13 protein, CI, and respirasomes) suggest that these novel variants in the <i>NDUFA13</i> gene are the underlying cause of the CI defect, expanding the genetic heterogeneity of LS.