PHF19 is a polycomb-group protein that recognizes histone H3 trimethylated at lysine 36 (H3K36me3) and recruits the PRC2 complex to enhance its H3K27me3 methylation activity, thereby mediating transcriptional repression. In embryonic stem cells, PHF19 facilitates the transition from active to repressed chr9 states by binding H3K36me3 and recruiting histone demethylases, which leads to PRC2 recruitment and de novo gene silencing. PHF19 dysregulation is implicated in multiple human cancers through context-dependent mechanisms. In high-risk multiple myeloma, aberrant PHF19 overexpression is the most significant contributor to treatment-resistant disease, where it modulates PRC2 activity and regulates genes controlling cell cycle and genetic stability 1. In colorectal cancer, PHF19 is governed by oncogenic super-enhancers and promotes tumorigenesis 2. In glioma, PHF19 overexpression activates EZH2 and drives aberrant proliferation, which can be suppressed by the inhibitory microRNA miR-124a 3. Recent evidence suggests context-specific roles: in advanced prostate cancer, a long isoform of PHF19 forms biomolecular condensates with YTHDC1 that paradoxically switch EZH2 from gene repression to activation 4, while in hepatocellular carcinoma, PHF19 promotes growth by stabilizing the Hedgehog pathway effector Gli1 5. Therapeutic targeting of PHF19 shows promise; inhibition suppresses leukemia and glioma cell growth in preclinical models 36, making PHF19 a candidate for epigenetic therapies in aggressive malignancies.