Peptides are small molecules that act like messengers, delivering signals by...

https://bizzmarkblog.com/how-do-researchers-know-which-receptor-caused-a-response/

Peptides are small molecules that act like messengers, delivering signals by binding to specific receptors on cells. Researchers use peptides to explore how receptors work, test which ones they bind to best, and map out the signaling pathways they trigger