GEOapplication/xmlftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE330nnn/GSE330516/primaryOK200TranscriptomicsHomo sapiensExpression profiling by high throughput sequencinghttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE330516GEOGSEfalseA non-canonical function of the tyrosine degradation enzyme FAH drives CDK4/6 inhibitor resistance in breast cancer IResistance to standard-of-care therapies remains a major clinical challenge in the treatment of the most common breast cancer, the hormone receptor–positive (HR+) subtype. Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors improve outcomes in early-stage HR+ disease, yet many patients relapse. Resistance mechanisms of relapsed tumors include genetic alterations, but in many cases no genetic drivers are identified. Here, we investigated mechanisms underlying resistance to CDK4/6 inhibitors using breast cancer patient-derived models and tumors. We identified an unexpected, non-catalytic nuclear function of fumarylacetoacetate hydrolase (FAH), an enzyme in the tyrosine catabolism pathway, as a driver of resistance. FAH translocated to the nucleus upon CDK4/6 inhibition, where it interacted with cyclin-dependent kinase 9 (CDK9), and promoted resistance. Nuclear FAH was enriched in tumors from relapsed patients, and inhibition of CDK9 reversed FAH-mediated resistance. These findings establish nuclear FAH as a biomarker of resistance and revealed CDK9 as a therapeutic vulnerability in CDK4/6 inhibitor-resistant HR+ breast cancer.2026/07/30GSE330516GSM9728031GSM9728020GSM9728030GSM9728022GSM9728033GSM9728032GSM9728021GSM9728024GSM9728034GSM9728023GSM9728026GSM9728025GSM9728028GSM9728027GSM9728019GSM972802924676330516Homo sapiens