Updated PubMed. Cancer cells undergo transcriptional reprogramming to drive tumor progression and metastasis. Here, we identified the transcriptional complex, NELF (Negative elongation factor), as an important regulator of this process. Using cancer cell lines and patient-derived tumor organoids, we demonstrated that loss of NELF inhibits breast cancer tumorigenesis and metastasis. Specifically, we found that epithelial-mesenchymal transition (EMT) and stemness-associated genes are downregulated in NELF-depleted breast cancer cells. Quantitative Multiplexed Rapid Immunoprecipitation Mass spectrometry of Endogenous proteins (qPLEX-RIME) of NELF-E, a key subunit of NELF, reveals significant rewiring of NELF-E-associated chromatin partners as a function of EMT, and further illuminates a co-option of NELF-E with the key EMT transcription factor SLUG. Accordingly, loss of NELF-E led to impaired SLUG binding on chromatin. Through integrative transcriptomic and genomic analyses, we identified the histone acetyltransferase, KAT2B, as a key functional target of NELF-E-SLUG. Genetic and pharmacological inactivation of KAT2B ameliorate expression of critical EMT marker genes, phenocopying NELF ablation. Elevated NELF-E and KAT2B expressions are associated with poorer prognosis in breast cancer patients, highlighting the clinical relevance of our findings. Importantly, KAT2B knockout mice are viable, raising the exciting prospect of targeting this dependency therapeutically. Taken together, we uncovered a crucial role of the NELF-E-KAT2B epigenetic axis in breast cancer carcinogenesis.