In this task, you are given a sentence. You are expected to recognize the name of any form of chemical substances. Although there might be several correct answers, you need to write one of them.

Example Input: Selective inhibition of cyclooxygenase 2 spares renal function and prostaglandin synthesis in cirrhotic rats with ascites.
Example Output: prostaglandin

Example Input: Efficient induction of apoptosis in HeLa cells by a novel cationic porphycene photosensitizer.
Example Output: porphycene

Example Input: Synthesis of novel estrogen receptor antagonists using metal-catalyzed coupling reactions and characterization of their biological activity.
Example Output:
estrogen