Novel protein interaction inhibitor of replication protein A (RPA), a protein involved in the ATR/Chk1 pathway. HAMNO selectively binds the N-terminal DBD-F domain of RPA70, effectively inhibiting critical RPA protein interactions that rely on this domain. HAMNO inhibits both ATR autophosphorylation and phosphorylation of RPA32 Ser33 by ATR. Candidate therapeutic for cancer treatment, as it enhances the constitutive and oncogene-induced replication stress in cancer cells.