Scientists at UC Davis discovered a small genetic difference that could explain why humans are more prone to certain cancers than our primate cousins. The change affects a protein used by immune cells to kill tumors—except in humans, it’s vulnerable to being shut down by an enzyme that tumors release. This flaw may be one reason treatments like CAR-T don’t work as well on solid tumors. The surprising twist? That mutation might have helped our brains grow larger over time. Now, researchers are exploring ways to block the enzyme and give our immune system its power back.