Final answer:
The PH (Pleckstrin Homology) domain is an example of a protein domain that is involved in cell-signaling pathways.
Step-by-step explanation:
A protein domain is a specific part of a protein that performs a specific function. In this case, the PH (Pleckstrin Homology) domain is an example of a protein domain. It is a common domain found in different proteins and is involved in binding a molecule called phosphatidyl-inositol triphosphate (PIP3). This domain allows the proteins to be a part of a cell-signaling pathway.