Co-expression of nuclear-coded mitochondrial genes. (a) Schematic diagram of the OXPHOS system showing the complexity of its subunit composition. IM: mitochondrial intermembrane space; M: mitochondrial matrix. (b) mRNA expression profiles for 1290 human and 1029 mouse mitochondrial compiled genes (rows) across 79 human (left) and 53 (right) mouse tissues (columns). Genes and tissues were hierarchically clustered and visualized using Genespring. Selected tissues are labeled (top). Clusters enriched in genes belonging to the same functional classification are labeled between brackets.