KAT6A is a lysine histone acetyltransferase (HAT) of the MYST family of HATs. KAT6A, and its paralog KAT6B, have been shown to acetylate histone H3K23Ac and regulate diverse biological processes, including transcription, cell-cycle progression, stem cell maintenance and development. Molecular dysregulation of KAT6A has been observed in several cancers, including amplifications in breast, lung, ovarian cancer along with oncogenic fusions in AML. In breast cancer, KAT6A is amplified as part of the 8p11 amplicon in 10-15% of the patient population, which correlates with a worse clinical outcome in the estrogen receptor+ (ER+) subtype. Here we present identification of a first-in-class potent KAT6A/KAT6B tool inhibitor CTx-648 (PF-9363), that possesses high selectivity versus other MYST family members (KAT7, KAT5, KAT8) and other KATs, demonstrating anti-tumor activity in breast cancer. Using genetic and pharmacological approaches, we have demonstrated several ER+ breast cancer cell lines including KAT6A amplified and over-expressing models, are dependent on KAT6A enzymatic function. Epigenomic profiling studies using bulk and nascent RNA-seq combined with ATAC-seq revealed CTx-648 leads to downregulation of a specific set of genes involved in ESR1 pathway, cell cycle and stem cell pathways. In vivo target validation studies showed strong anti-tumor activity of CTx-648 in several ER+ breast cancer cell line and patient-derived xenograft models, including models harboring endocrine therapy resistance ESR1 mutations, highlighting promise for this novel therapy in ER+ breast cancer population. Based on the strength of the pre-clinical data, a selective KAT6 inhibitor (PF-07248144) is now commencing a Phase 1 clinical study in Advanced or Metastatic Solid Tumors.