For the first time ever, biologists have regenerated an organ in a living animal. They did so by manipulating a single protein in elderly mice that makes their bodies rebuild their thymuses, an organ of the immune system. It's the first time an organ has been repaired with a chemical trigger and not via stem cell regeneration.
The bad thing about thymic involution is that it reduces your naive T cell repertoire, which makes you worse at fending off new infections or cancers as you age. So reversing it could be pretty important at preventing age-related cancers or opportunistic infections.
One nitpick: It's only a chemical trigger since they used a transgenic mouse with tamoxifen induced expression of FOXN1 (a transcription factor). So it still requires pretty large genetic manipulation.