This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Protein targeted for destruction in eukaryotes are covalently linked to 
Select and explain. 
A. Clathrin, B. Pepsin, C. Laminin, D. Ubiquitin
D: Ubiquitin
Answer: D: Ubiquitin 
Explanation: Ubiquitin plays a major role in the degradation of proteins and is particularly associated with the disposal of misfolded proteins.
It is a small, highly conserved protein that plays a key role in marking various proteins for subsequent degradation in proteosomes.