import string
from typing import Counter


def letter_frequency(seq):
    """Returns a dictionary with the frequencies of letters in the sequence"""
    freq = filter(lambda x: x in string.ascii_letters, seq.lower())
    freq = dict(Counter(freq).most_common())
    freq.update(dict((x, 0) for x in filter(lambda x: x not in freq, string.ascii_lowercase)))

    return freq