GEOapplication/xmlftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE311nnn/GSE311215/primaryOK200TranscriptomicsHomo sapiensExpression profiling by high throughput sequencinghttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE311215GEOGSEfalseTranscriptomic profiling in estrogen receptor-positive breast cancer cell lines CAMA-1 and ZR-75-1 following CDK4/6 inhibitor treatmentThe retinoblastoma protein (Rb) is a tumour suppressor best known for repressing E2F transcription factors and halting cell cycle progression. In hormone receptor-positive (HR+) breast cancer, CDK4/6 inhibitors activate Rb by preventing its phosphorylation, forming a key component of current endocrine therapy regimens. How pharmacologically activated Rb remodels chromatin and influences transcription beyond cell cycle arrest remains poorly understood. Here we show that CDK4/6 inhibition induces redistribution of hypo-phosphorylated Rb to promoters and enhancers. While Rb predictably binds to cell cycle gene promoters to repress transcription, at other sites it unexpectedly promotes expression of oestrogen-responsive genes by integrating into oestrogen receptor (ER)-rich transcriptional hubs. CDK4/6 inhibition enhances ER target gene expression in breast cancer cells, patient-derived xenografts, and clinical HR+ breast cancer samples in an Rb-dependent manner. This reprogramming is mediated in part by KDM5A, whose interaction with Rb contributes to gene regulation at these loci. Critically, components of this Rb-driven ER transcriptional program are pro-proliferative. In endocrine-sensitive tumours, this can be neutralised with anti-oestrogen therapy, explaining therapeutic synergy. In endocrine-resistant settings such as ESR1-mutant breast cancer, the program persists, limiting therapeutic efficacy. These findings reframe Rb as a dual-function transcriptional regulator that, while enforcing cell cycle arrest, can also activate programs that counteract its tumour suppressor function.2026/06/29GSE311215GSM9321208GSM9321209GSM9321202GSM9321203GSM9321211GSM9321200GSM9321201GSM9321206GSM9321207GSM9321204GSM9321205GSM9321194GSM9321195GSM9321198GSM9321199GSM9321210GSM9321196GSM932119718573311215Homo sapiens