Polymorphic viruses create varied (though fully functional) copies of themselves as a way to avoid detection by anti-virus software. Some polymorphic virus use different encryption schemes and require different decryption routines. Thus, the same virus may look completely different on different systems or even within different files. Other polymorphic viruses vary instruction sequences and use false commands in the attempt to thwart anti-virus software. One of the most advanced polymorphic viruses uses a mutation engine and random-number generators to change the virus code and its decryption routine. Also see: mutating virus.