PTK7 and colorectal carcinoma: For example, CD133, a transmembrane glycoprotein, is associated with tumor-initiating cells (CIC) of malignant behavior [5] and protein-tyrosine pseudokinase 7 (PTK7), a component of the Wnt signaling pathway, is expressed in stem cells of the human colon [6] and marks a CRC cell population with motility and metastasis [7].