Is It DNA?

Is it DNA? DNA had four types of bases A,C,G,T these combine to make your DNA. In this task the user had to enter a string and the program had to do nothing if the string was correct and list any characters that weren't valid as DNA. In this programming I used for in dna and then an if statement in it with if not in 'ACGT'. Then printed "is not valid DNA. This was not to hard coding but it was simply getting the lopping right.