GEOapplication/xmlftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE303nnn/GSE303330/suppl/filelist.txtftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE303nnn/GSE303330/suppl/GSE303330_RAW.tarftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE303nnn/GSE303330/primary200OKGenomicsHomo sapiensGenome binding/occupancy profiling by high throughput sequencinghttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE303330GEOGSEfalseRb CUT&RUN profiling in MCF7 parent and RB1 knockout cells following abemaciclib 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.2026/06/28GSE303330GSM9123604GSM9123595GSM9123598GSM9123599GSM9123596GSM9123597GSM9123602GSM9123603GSM9123600GSM912360118573303330Homo sapiens