Metamorphic code is used by computer viruses to avoid the pattern recognition of anti-virus software.
Metamorphic viruses often translate their own binary code into a temporary representation, editing the temporary representation of themselves and then translate the edited form back to machine code again.
The mutated code will do exactly the same thing (under the interpretation used), but the child's binary representation will typically be completely different from the parent's.
The beginning of the virus is then coded so that it translates to correct machine-code for all of the platforms that it is supposed to execute in.
[2] This is used primarily in remote exploit injection code where the target platform is unknown.