Instead, the molecular recognition of estrogens (C3 phenol) over progestins (C3 ketone) uses a network of H-bonds with a phenol that cannot be achieved by a ketone. Nonsteroidal estrogens such as the phenolic thieno [2,3-d]pyrimidines have more options in how they bind and stabilize the receptor and a single H-bond is apparently sufficient to produce a stable ligand-receptor complex.