Number one is that they live where they're expected to live. They live just underneath the surface of the bone, underneath cartilage. You know, in biology, it's location, location, location. And they move into the appropriate areas and form bone and cartilage. That's one. Here's an interesting property. You can take them out of the vertebrate skeleton, you can culture them in petri dishes in the laboratory, and they are dying to form cartilage. Remember how we couldn't form cartilage for love or money? These cells are dying to form cartilage.