def is_nucleotide(sequence):
    """
    Checks that the string sequence provided is a valid string
    consisting only of the 4 nucleotides A, C, G, and T
    Returns True if so, and False otherwise

    :param sequence: a DNA sequence
    :return: A boolean value indicating if the sequence is valid
    """
    valid_string = ["A", "C", "G", "T"]
    for letter in sequence:
        if letter not in valid_string:
            return False
    return True