The S-enantiomer of A66 is a potent inhibitor of the p110alpha isoform of PI 3-kinase (PIK3CA). It also inhibits the E545K and H1047R mutant forms of PIK3CA which are the most common oncogenic forms of this gene. A66 is more than 100 fold less active against the other class-I PI 3-kinase isoforms and had not inhibitory activity against 200 protein kinases when tested at 10 micromolar. This makes it the most selective and specific p110alpha inhibitor available for research purposes