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Analysis of amino acid composition heterogeneity of mitochondrial proteins among metazoans. Principal Component Analysis (PCA) of the amino acid composition of the 13 mitochondrial proteins from the 54-taxon metazoan dataset. Individuals are plotted in the first two principal components of the PCA which explain 53.1% and 15.3% of the total compositional variance, respectively. Points corresponding to Cephalochordata (red), Tunicata (purple) and Gnathostomata (blue) are circled.