For the first time, a pig kidney has been transplanted into a human without triggering immediate rejection by the recipient's immune system. It is a potentially major advance that could eventually help alleviate a dire shortage of human organs for transplant. The pig's genes were altered so its tissues no longer contained a molecule known to trigger almost immediate rejection. The recipient was a brain-dead patient with signs of kidney dysfunction whose family consented to the experiment before she was due to be taken off life support. More than 90,000 people in the US are awaiting a kidney, with wait times for averaging three-to-five years.