ABSTRACTCTR9 is the scaffold subunit in Paf1c, a multifunctional complex regulating multiple steps of RNA Pol II-mediated transcription. Using inducible and stable CTR9 knockdown breast cancer cell lines, we discovered that the expression of a subset of KDMs, including KDM6A and Jarid2, is strictly controlled by CTR9. Global analyses of histone modifications revealed a significant increase of H3K27me3 upon loss of CTR9. Loss of CTR9 results in a decrease of H3K4me3 and H3K36me3 in gene bodies, and elevated levels and genome-wide expansion of H3K27me3. Mechanistically, CTR9 depletion triggers a PRC2 subtype switching from PRC2.2 to PRC2.1. As a consequence, CTR9 depletion generates vulnerability that renders breast cancer cells hypersensitive to PRC2 inhibitors. Our findings that CTR9 demarcates PRC2-mediated H3K27me3 levels and genomic distribution, provide a unique mechanism of transition from transcriptionally active to repressive chromatin states and sheds light on the biological functions of CTR9 in development and cancer.