Many of the 17,000 people currently waiting for liver transplants in the U.S. have had bags packed for weeks, months, or even years, ready for the moment the phone rings and the voice on the other end tells them to head to the hospital. Once an organ becomes available, they know, there will only be a small window in which they can receive it. This year, an estimated 6,000 of those people will undergo a transplant; another 1,500 will die waiting for that call. There just aren't enough livers to go around.