You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
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.

Inhibition of the human ether-a-go-go-related gene (HERG) potassium channel by cisapride: affinity for open and inactivated states.
Solution: potassium
Why? Potassium is a type of electrolyte mineral that helps our nerves to function and muscles to contract. Hence, it should be tagged as chemical.

New input: PPARgamma controls CD1d expression by turning on retinoic acid synthesis in developing human dendritic cells.
Solution:
retinoic acid