MMP14 and cancer: Ubiquitous membrane type-1 matrix metalloproteinase (MT1-MMP), arguably the primary enzyme in pericellular proteolysis and cancer cell migration, cleaves the PKP621↓LI sequence in the seventh Ig-like domain of membrane PTK7 and this cleavage results in the liberation of the N-terminal soluble PTK7 fragment (sPTK7) [21].