The study, which looked at 186 patients, found that fixing the CDKN1B gene could help tamoxifen work again in those who'd become resistant.
Even better, another group of drugs called CDK4/6 inhibitors (like palbociclib) seems to work even when p27 is missing.
Researchers think checking p27 levels could help doctors personalize treatments for patients with low p27 levels or at high risk of resistance.