def nucleotides_to_integers(seq):
    """Converts a nucleotide sequence to an interger representation.

    The base characters in the nucleotide string (A, C, G and T) are converted to the following: A -> 0, C -> 1, G -> 2
    and T -> 3. The combined uppercase string is returned.

    Parameters
    ----------
    seq : str
        A nucleotide sequence string.

    Returns
    -------
    str
        The interger representation string for the given nucleotide sequence.

    """
    int_sequence = []
    for i in seq.upper():
        if i == 'A':
            int_sequence.append(str(0))
        elif i == 'C':
            int_sequence.append(str(1))
        elif i == 'G':
            int_sequence.append(str(2))
        elif i == 'T':
            int_sequence.append(str(3))
    return ''.join(int_sequence)
