Remember how impermeable that basal lamina looked in today's lesson's electron micrograph of a torn epithelium? Cancer cells can actually break right through such barriers, to gain access to the circulatory system and begin a metastatic journey.
How do they break through? Matrix metalloproteinases (MMPs) are proteins that when bound to a metal (such as zinc) can break down other proteins. They are typically secreted into the extracellular space.
MMPs weren't "invented" for cancer, though; they're important for development, when cells need to break from one compartment of the body to another in order to set up the body plan and build organs.
MMPs are seen in the genomes of organisms besides humans (and related mammals) - they're expressed by venomous snakes! It turns out snake venom is so destructive because it contains MMPs. Venoms can induce much of their harm by causing hemorrhage (breaking down blood vessels and inhibiting clotting).
There is a whole field of Venomics (get it? Like genomics, transcriptomics, proteomics, etc) in which folks are working to better understand the molecular and cellular biology and the biochemistry of venom proteins including MMPs in order to engineer them for therapies, develop anti-venom treatments, and widen our understanding of the diversity of protein function.