PTK2 and colorectal cancer: 5‐HT activates 5‐HTRs to initiate Wnt/b‐catenin signaling in order to promote the self‐renewal of colorectal cancer stem cells, integrin/Src/Fak‐mediated signaling, uPAR/MMP‐2 signaling, and zinc finger ZEB1 and Snail protein expression.