24 Hormone receptor positive breast cancer, the most common subtype of the disease, accounting for around 70% of cases, often develops resistance to standard therapies, posing a major challenge for patients and clinicians. A new study by researchers at Boston Children’s Hospital and colleagues has uncovered a previously unknown driver of this resistance, offering potential avenues for earlier detection of resistant cells and targeted interventions. Hormone receptor positive breast cancers rely on hormones such as oestrogen or progesterone to fuel their growth. The current standard of care combines endocrine therapy with CDK4/6 inhibitors, which work by blocking enzymes that promote cell cycle progression, thereby slowing tumour proliferation. While these drugs have significantly improved progression-free survival for many patients, acquired resistance remains a common barrier to long-term disease control. Many patients eventually develop resistance, allowing cancer cells to regrow and leading to poorer treatment outcomes. Much remains unknown about the genetic drivers responsible. In results published today in Science Advances, a research team co-led by Naama Kanarek, PhD, in the Pathology Department at Boston Children’s, and Taru Muranen, PhD, identified an enzyme called fumarylacetoacetate hydrolase, or FAH, as a key player. Long thought to function strictly as a metabolic enzyme, FAH was found to possess a second role that drives a novel resistance mechanism. Previously known only for its activity in the cell’s cytosol, where it participates in tyrosine breakdown, FAH was also found in the nucleus of breast cancer cells treated with CDK4/6 inhibitors. The team further elucidated the molecular mechanism underlying FAH-mediated resistance. They demonstrated that FAH interacts with the cell fate regulator CDK9 inside the nucleus and stimulates its activity. Importantly, inhibiting CDK9 was shown to reverse the resistance driven by FAH. The findings raise the possibility of using nuclear FAH as a biomarker to identify resistance to CDK4/6 inhibitors at an earlier stage. This could allow clinicians to detect resistant cancer cells before full clinical relapse occurs, enabling more personalised monitoring and timely adjustments to therapy. The findings also point to a potential clinical strategy. CDK9 inhibition, alone or in combination with CDK4/6 inhibitors, may help mitigate relapse in patients with hormone receptor positive breast cancer receiving or having received CDK4/6 inhibitor therapy. Because the data suggest that this pathway is directly involved in mediating resistance, targeting it could potentially restore sensitivity in cells that have adapted to CDK4/6 blockade. However, these approaches would require further research to determine whether they are safe and effective in patients. The discovery that a metabolic enzyme can relocate to the nucleus and influence cell fate regulators such as CDK9 highlights the complex adaptations cancer cells can undergo under therapeutic pressure. It also shows how enzymes traditionally associated with metabolism may contribute to therapeutic escape when they operate in different cellular compartments. Further research may help refine biomarker assays and evaluate the safety and efficacy of CDK9-targeted approaches in patients with hormone receptor positive disease.