The absent, small, or homeotic-like 1 (ASH1L) protein is a histone lysine methyltransferase that plays a crucial role in various cancers, including leukemia. Despite representing an attractive therapeutic target, only one class of ASH1L inhibitors was identified to date. Herein, we report development of advanced ASH1L inhibitors targeting the catalytic SET domain, which were designed to access previously unexplored binding pocket on ASH1L. Extensive medicinal chemistry combined with structure-based design led to identification of