Doctors in Serbia have accomplished a rare feat in the field of transplants: They removed a testicle from a donor and implanted it into his identical twin brother, reports the New York Times. The 36-year-olds are doing fine, and the procedure should allow the recipient not only to have more stable levels of testosterone and natural-looking genitals, but to father children. Of course, the latter point raises an ethical question that speaks to why procedures of this sort aren't as common as they might be: "Whose offspring are they—his, or the donor's?" as London's Sunday Times put it in an earlier story on the general topic. This is the third known testicle transplant, the previous two having been done 40 years ago in the US, also between identical twins, notes the New York Times.
One huge advantage with twins: Because the donor and recipient share their genetic makeup, the recipient doesn't need to take immune-suppressing drugs. In the Serbian case, the recipient was born without testicles, and while that's rare, such an operation might be feasible for transgender patients or those injured in accidents or war. However, the ethical issues involved make it tricky. For example, when a wounded soldier had a successful penis transplant, the donor didn't agree to donate his testicles as well, because any offspring might be considered the donor's genetic children. (In another notable transplant case, a woman met the man who received her husband's face.)