MUC4 and periodontitis: The most highly up-regulated gene was MUC4 and the second was MMP7. Further characterization of the association between periodontitis and the chronic inflammatory disorders CVD, RA, and UC identified only one gene, pleckstrin (PLEK), that was commonly up-regulated in all four diseases, suggesting this gene as a key link between periodontitis and various systemic diseases associated with periodontitis.