Running a for loop only after an if condition has been met

I'm currently trying to make a very simply testing function that first checks to see if the users input is a multiple of 3, then tests each individual character to see if they are valid characters. Here's my code below:

def is_dna(string):

string.upper() 
if(len(string) % 3 == 0):
    print("LENGTH CORRECT")
    for n in string:
        if(n == "A" or n == "T" or n == "C" or n == "G"):
            print("Valid")
        else:
            print("Invalid character")
            break
    return True
else:
    print("Too many/little characters")
    return False

When run, the bottom section will run fine, and if a correct amount of characters is used this will also successfully print the debug "LENGTH CORRECT" string. The issue is that the for loop will not initialize, and I haven't the foggiest why. Testing just the loop shows it to work fine; what is wrong with this function?