Thick bones, big drug

It was 1994, and Scott Simonet of Amgen's molecular genetics department in Thousand Oaks, Calif., was looking at some strange X-rays. He had engineered five transgenic mouse lines to overexpress a mysterious secreted protein. The mice looked and behaved normally, but that ordinariness was only skin-deep. "On the X-rays, it was pretty obvious that the long bones had higher bone mineral density," says Simonet. The pelvic and vertebral bones also appeared thicker than usual. It was clear that the mysterious protein played a role in building bone.