Representation of a network of protein-protein interactions and its correlation with mRNA expression profiles. The lower left triangle corresponds to a network of protein-protein interactions derived from small-scale experiments between 58 cell cycle regulators. For each two regulatory proteins, an association value was calculated as 1/d2, where d is the shortest path between the proteins in the interaction network . The upper right triangle corresponds to correlation of mRNA expression profile changes during the cell cycle between pairs of cell cycle regulatory genes. Protein pairs that are colored red correspond to Pearson correlation coefficient (P) values greater than 0.5, and those colored green correspond to P values less than -0.5. In the bottom left triangle, an association value of two proteins that have a significant positive correlation coefficient in their mRNA expression levels is colored magenta (for positive correlation) or cyan (for negative correlation). Association values between uncorrelated proteins are colored blue.