Montgomery was born with a heart condition that killed both his father and older brother, both of whom died young (his brother at 35, his dad at 52). He finally got a heart transplant in 2018, after years of waiting because he wasn't "sick enough" to make the organ donor list.

So he knows all too well "what the waiting is like as a patient," Montgomery, head of the NYU Langone Transplant Institute, told The Post. "The uncertainty of not knowing if you're going to get an organ. I'm very aware of the people who don't make it across the finish line."

Although his patient was clinically brain-dead before the operation, the transplanted kidney remained functional for 54 hours, long enough to detect any immediate rejection. It's a promising sign that xenotransplantation — the medical term for implanting other species' organs and tissues into humans — may soon become the norm.