def METHOD_NAME(mRNA):
    polypeptides = mRNA.polypeptides()
    if len(polypeptides) == 0:
        polypeptide_id = "{0}.polypeptide".format(mRNA.id)
        polypeptide = things.Polypeptide(id=polypeptide_id, parent=mRNA.id)
        mRNA.add_polypeptide(polypeptide)