def is_valid_nuc(nuc):
    """Check if valid single letter base.

    Parameters
    ----------
    nuc : str
        String to check if valid single nucleotide base

    Returns
    -------
    is_valid : bool
        flag indicating valid nucleotide base
    """
    valid_nucs = ['A', 'C', 'T', 'G', 'N']
    is_valid = nuc in valid_nucs
    return is_valid