import string


def isPalindrome(s: str) -> bool:
  """
  s = string
  Ignore string cases & non-letters.

  Returns True if all letters in s form a palindrome, otherwise returns False.
  """

  s = s.lower()
  letters = ""
  for char in s:
    if char in string.ascii_lowercase:
      letters += char

  def isPal(s: str) -> bool:
    if len(s) <= 1:
      return True
    else:
      return s[0] == s[-1] and isPal(s[1:-1])

  return isPal(letters)