Dec. 9, 2019 -- A man who was born without testicles got a transplanted one from his identical twin brother, doctors say.

The six-hour operation on the 36-year-old patient was performed early last week in Belgrade, Serbia, by an international team of surgeons, The New York Times reported.

Being born without testicles is rare and this is only the third known transplant of its type. The first two were performed in St. Louis 40 years ago and involved two pairs of identical twin brothers. In each pair, a brother did not have testicles.

The goals of the recent surgery included giving the patient more stable levels of the male hormone testosterone than could be provided by injections and to enable him to father children, Dr. Dicken Ko, a transplant surgeon and urology professor at Tufts University School of Medicine in Boston, told The Times.

Ko was one of the surgeons on the transplant team.

By last Friday, the recipient had normal testosterone levels, Ko told The Times.

Surgeons operated on the brothers simultaneously, in adjoining rooms, the newspaper reported. During the intricate procedure, doctors had to stitch together two arteries and two veins that were less than 2 millimeters wide.

"Once you remove the testicle from the donor, the clock starts ticking very fast," Dr. Branko Bojovic, an expert in microsurgery at Harvard Medical School who participated in the surgery, told the The Times.

"Within two to four hours, you have to have it re-perfused and working again," Bojovic said. Without a blood supply, a testicle is viable for only four to six hours.

Doctors say testicle transplant surgery could have wider applications for transgender people, accident victims, wounded soldiers and cancer patients, The Times reported.