Gene expression data Principal Component Analysis (PCA) scatterplots. showing PC1 vs PC2 (left) and PC3 vs PC4 (right). Individual samples are represented by dots. The percentage of total variation explained by each component is given in the x and y-axis as appropriate. To control over representation of very highly expressed genes, all gene expression values were scaled on a gene by gene basis using the z-score transformation, prior to the PCA.  
