In addition to the large spectrum of the protein precursors processed and activated by the proprotein convertases (PCs) that are crucial for the maintenance of the malignant phenotype of colon cancer cells such as matrix metalloproteases, adhesion molecules, growth factors, and growth factor receptors, the PCs also regulate the expression and the activity of other proteins that are not PC substrates and involved in the acquisition of the metastatic and tumorigenic potential of these tumor cells. The identification in colon cancer cells of such proteins is thereby crucial for the understanding of the cascade of molecular events regulated by the PCs leading to tumorigenesis and metastasis and thus may constitute potential candidates for new colon cancer-specific targets and/or biomarkers. Using the human colon cancer cells HT-29 and ProteinChip arrays analysis that apply the surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF-MS), we identified the myosin heavy polypeptide 9 as new downstream effector of PCs in these cells. This protein was reported to be involved in the processes of malignant epithelial transformation and its role in colon cancer is unknown.