The expression of a cloned eukaryotic gene in a bacterial host cell is challenging due to the following factor(s):

• A. Presence of introns in eukaryotes.
• B. Differences in promoters and other DNA control sequences between prokaryotes and eukaryotes.
• C. Post-transcriptional and post-translational modifications in eukaryotes.
• D. All of the above.

Answer: (D)