def is_valid_seq(seq, max_len=2000):
    """
    True if seq is valid for the babbler, False otherwise.
    """
    l = len(seq)
    valid_aas = "MRHKDESTNQCUGPAVIFYWLO"
    if (l < max_len) and set(seq) <= set(valid_aas):
        return True
    else:
        return False