import re


def cpg_map(seq):
    """ Return tuple of C/G/N.

    >>> cpg_map('CGCGTAGCCG')
    'CGCGNNNNCG'
    """
    starts = (x.start() for x in re.finditer('CG', ''.join(['N', seq, 'N'])))
    cpgs = ['N'] * (len(seq) + 2)
    for start in starts:
        cpgs[start] = 'C'
        cpgs[start+1] = 'G'
    return ''.join(cpgs[1:-1])