def translate_codon(codon):
    """Convert a codon (three nucleotides) to the corresponding protein."""
    # Make sure that we have a length three string
    if not(is_string(codon)):
        parameter_error("translate_codon", 1, "string", codon)
    elif len(codon) != 3:
        parameter_error("translate_codon", 1, "length 3 string", codon)
    # For convenience, put it into uppercase
    codon = codon.upper()
    # Look it up in the table
    try:
        return codons[codon]
    except KeyError:
        return '*'