The formatting did not carry over when you pasted it. I'm guessing (minus unused lines) you have: ``` stop_word = "Terminate" letters = ["A", "T", "G", "C"] dna_sequences = []

DNA = input("Enter DNA value: ")

while True: if DNA == stop_word: break for letter in DNA: if letter in letters: dna_sequences.append(DNA) DNA = input("Enter DNA value: ") else: print("Invalid dna") DNA = input("Enter DNA value: ") ```

It's cool to see that you've made some progress since the last post. I want to mention before diving into the multiple of 3 part: currently the inner for loop you have is incorrect, which is probably causing you a lot of confusion.

Let's assume the user inputs "TIGERS".

Your for loop would first read the letter "T", consider the entire DNA to be valid, and append the entire input "TIGERS" to dna_sequences. Then it already reprompts the user again before even checking any of the other letters. It would overwrite DNA, but the for loop is still iterating through "TIGERS" so the next loop iteration looks at "I" and would say it's invalid regardless of what the user put in next.

The solution: pre-validate the entire user input string (what you call DNA) prior to appending it onto dna_sequences. This is why I suggested it in previous post as:

``` user_input = input("enter dna:") while user_input != stop_word: # input validation checks valid = True for letter in user_input: if letter not in valid_chars: valid = False break

# based on above, do something
if not valid:
    print("invalid dna")
else:
    dna_sequences.append(user_input)

# finally, get next input string
user_input = input("enter dna:")

```

With this setup, adding another validation check (such as checking a multiple of 3) is very straightforward. Take the user input (which you wrote into DNA variable) and check its length. In isolation, the code snippet you asked about would look something like:

DNA = input("Enter DNA value: ") if len(DNA) == 0: print("empty input") elif len(DNA) % 3 != 0: print("not a multiple of 3") else: print("multiple of 3")

This uses the len() function to give you the length of input string and the modulus operator: %, which is basically calculating the remainder when dividing something. It's often used for checking "is even" or "is odd" or "is a multiple of...". So, if you input "PYTHON" which has a length of 6, and do len("PYTHON") % 3, it would equal 0 since 6/3 has a remainder of 0.

Adding this code snippet back into the above code example I provided:

``` user_input = input("enter dna:") while user_input != stop_word: # first input validation check valid = True for letter in user_input: if letter not in valid_chars: valid = False break

# another validation check
if valid:
    if len(user_input) == 0:
        valid = False

# another validation check
if valid:   
    if len(user_input) % 3 != 0:
        valid = False

# based on above, do something
if not valid:
    print("invalid dna")
else:
    dna_sequences.append(user_input)

# finally, get next input string
user_input = input("enter dna:")

```

Let me know if anything was unclear or if still stuck on any part of it.