No matter where translation begins, only one polypeptide sequence pattern can be obtained from the translation of an mRNA chain with the sequence (AC)n; that is, an mRNA with the sequence 5'...ACACACACAC...3'. In contrast, an mRNA chain with the sequence (ACG)n can be translated to polypeptide sequences of three distinct patterns, depending on the choice of the reading frame. Using similar judgment, how many different polypeptide sequence patterns can be obtained from an mRNA chain with the sequence (ACGU)n? Write down your answer in digits, e.g. 12.