Also know as mutating or self-encrypting virus, a virus that can change its byte pattern when it replicates, avoiding detection by simple string-scanning techniques. Some polymorphic viruses use different encryption schemes and require different decryption routines. Thus, the same virus may look completely different on each system or even within separate files. Other polymorphic viruses vary instruction sequences and use false commands in an 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.