def validate_sequence_characters(sequence):
    """
    Validates that the sequence passed into it consists only of A, T, G and Cs.
    """
    sequence = set(sequence.upper())
    allowed = ['A', 'T', 'G', 'C']
    for s in sequence:
        try:
            assert s in allowed
            return True
        except AssertionError:
            return False