On the right, you can clearly see a bone cavity in the X-ray of a normal cat. But, in Lil Bub's X-ray on the left, you see mineralized bones that are very thick at the ends instead of a normal bone cavity.
This is called Erlenmeyer flask bones, and some humans also have the condition, according to the project's page. The scientists want to know if there's a specific mutation that causes the condition in humans and cats.