LRE1 is a selective allosteric inhibitor of soluble adenylyl cyclase (sAC), a ubiquitously expressed, essential component of cAMP-signaling. In contrast to G-protein-regulated transmembrane adenylyl cyclase, soluble adenylyl cyclase is directly activated by calcium and carbonate, and is dispersed throughout the cell cytoplasm. LRE1 was found to bind to the bicarbonate activator binding site. It inhibited cAMP accumulation in 4-4 cells with an IC50 of 11 µM, similar to that of KH7 and without its cellular toxicity.