An in vitro translation system can synthesize peptides in all three reading frames of the RNA template. When 5' - UCUCUCUC-(UC)<sub>n</sub>-UCUCUCUC - 3' was used as the template in this in vitro translation system, the synthesized peptides contained 50% each of serine and leucine. When 5' - CCUCCUCCU-(CCU)<sub>n</sub>-CCUCCU - 3' was used as the template, the synthesized peptides contained 33.3% each of serine, leucine, and proline. Deduce the codon for proline.

Correct Answer: CCU