Final answer:
The outside of RPTKs is ligand binding and highly variable; and the inside near the TK domain are highly conserved.
Step-by-step explanation:
The outside of receptor protein tyrosine kinases (RPTKs) is ligand binding and highly variable; and the inside near the tyrosine kinase (TK) domain is highly conserved.
This means that the external part of RPTKs, called the extracellular domain or ligand-binding domain, can interact with different ligands in a diverse manner. On the other hand, the internal part of RPTKs, closer to the TK domain, remains similar across different receptors and plays a crucial role in phosphorylation and signal initiation within the cell.