A new study has found an unexpected culprit behind breast cancer drug resistance — a metabolic enzyme called FAH hiding in the cell nucleus. Here is what the research shows.

**The discovery.** Scientists at Boston Children’s Hospital, led by researchers Kanarek and Muranen, found FAH inside the nucleus of hormone-receptor-positive (HR+) breast cancer cells being treated with CDK4/6 inhibitors. The study was published in Science Advances.

**Why it matters.** CDK4/6 inhibitors are a mainstay of treatment for HR+ breast cancer, the most common subtype. But tumours often stop responding. The team found that nuclear FAH activates a protein called CDK9, which helps cancer cells survive the drugs — a previously unknown resistance pathway.

**The hope.** In lab experiments, blocking CDK9 reversed the resistance, pointing to a possible new combination therapy: CDK4/6 inhibitors plus CDK9 blockers. FAH could also serve as a biomarker to predict whose cancer will stop responding.

**The caveat.** These are laboratory findings. Clinical trials will be needed to prove the strategy works in patients — but for a disease where resistance is one of the biggest challenges, it is a promising lead.