That something, the team showed, is interacting with a protein called CDK9, a regulator of cell fate. In the nucleus, FAH stimulates CDK9's activity, and it is this stimulation that helps cancer cells survive and keep dividing despite CDK4/6 inhibitor treatment. In other words, FAH acts as an accomplice: the drugs shut one door to cell division, and nuclear FAH helps pry open another.

Crucially, the researchers tested whether blocking this second pathway could undo the resistance. When they inhibited CDK9, FAH-mediated resistance was reversed. That result, obtained in laboratory models, suggests a direct therapeutic strategy: combining CDK4/6 inhibitors with drugs that block CDK9 could, in principle, prevent or overcome resistance in patients.

The findings point to two practical applications, both of which will require further research before reaching the clinic. First, the presence of FAH in the nucleus could serve as a biomarker — a measurable biological signal — that flags cancer cells that are becoming, or have already become, resistant to CDK4/6 inhibitors. Detecting resistance early would allow doctors to change treatment before the cancer regrows visibly. Second, CDK9 inhibition, used alone or alongside existing CDK4/6 inhibitors, could offer a way to reduce relapse in patients with HR+ breast cancer.

There are important caveats. The study's results come primarily from laboratory and cellular models, and the pathway from a mechanistic discovery to an approved therapy is long. The researchers have not yet demonstrated that nuclear FAH reliably predicts resistance in a large cohort of patients, nor have CDK9 inhibitors been tested in combination with CDK4/6 inhibitors in this specific clinical setting. Whether the benefits observed in cells will translate to people remains an open question.

Still, the discovery is significant because it changes how scientists understand both FAH and the biology of treatment resistance. Enzymes are often assumed to have one defined role, defined by where they act and what chemical reactions they catalyze. FAH now joins a growing list of proteins with "moonlighting" functions — second jobs that emerge under specific conditions, such as when a cancer cell is under attack from a targeted drug.

For the many HR+ breast cancer patients who eventually face drug-resistant disease, the study offers a new molecular thread to pull. If nuclear FAH proves to be a reliable early warning sign in follow-up studies, and if CDK9-blocking strategies hold up in further testing, clinicians may one day have both an earlier way to detect trouble and a targeted way to address it.

The paper, "A non-canonical function of the tyrosine degradation enzyme FAH drives CDK4/6 inhibitor resistance in breast cancer," by Jenny Högström and colleagues, appears in Science Advances (2026), DOI: 10.1126/sciadv.aef1145.