def retrieve_iupac(nucleotide):
    """
    Params:

    * *nucleotide (str)*: single character

    Returns:

    * *iupac_tuple (tuple)*: tuple of strings with possible bases
    """
    iupac_key = {'R': ('A', 'G'), 'Y': ('C', 'T'), 'S': ('G', 'C'), 'W': ('A', 'T'), 'K': ('G', 'T'), 'M': ('A', 'C'),
                 'B': ('C', 'G', 'T'), 'D': ('A', 'G', 'T'), 'H': ('A', 'C', 'T'), 'V': ('A', 'C', 'G'),
                 'N': ('A', 'C', 'G', 'T')}
    try:
        iupac_tuple = iupac_key[nucleotide]
    except KeyError:
        iupac_tuple = tuple(nucleotide)
    return iupac_tuple