def common_letters_in_IDs_differing_with_one_letter(boxes):
    """Find common letters in two IDs, which are differing with one letter."""
    for index, box in enumerate(boxes[:-1]):
        for other_box in boxes[index + 1:]:
            same_letters = [letter_a
                            for letter_a, letter_b in zip(box, other_box)
                            if letter_a == letter_b]
            if len(box) - 1 == len(same_letters):
                return ''.join(same_letters)
    return None