CTR9 is a scaffold component of the PAF1 transcription elongation complex that associates with RNA polymerase II to regulate transcriptional elongation and histone modifications 1. The complex recruits histone-modifying enzymes, including the RNF20/40 complex for H2B ubiquitination and SET1-related complexes for H3K4me3 deposition, processes essential for transcription activation 2. CTR9 also negatively regulates PRC2-mediated H3K27me3 repressive chr11 marks, demarcating active transcriptional domains 3. Disease relevance is substantial across multiple pathologies. Loss-of-function CTR9 variants increase myeloid malignancy risk ~10-fold by expanding hematopoietic stem cells through altered super elongation complex activity 4. De novo CTR9 missense variants cause neurodevelopmental disorders with macrocephaly, motor delay, and intellectual disability through loss-of-function mechanisms 5, and CTR9 is identified among high-confidence novel neurodevelopmental disorder genes 6. Conversely, CTR9 overexpression promotes glioma progression via JAK2/STAT3 pathway activation 7, and CTR9 protein stabilization drives hepatocellular carcinoma epithelial-mesenchymal transition 8. These bidirectional roles suggest CTR9 function must be precisely balanced for normal development and cancer prevention.