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Mirror life is a type of lifeform that uses the same type of DNA structure, but where the DNA is left-handed. This type of life, in turn, used right-handed amino acids.
Why does the difference matter? If single cell members of mirror life (with left-handed DNA) are placed in a nutrient broth consisting of only left-handed amino acids, the lifeforms will not be able to thrive. The same is true of the reverse.
The possibly that life on other worlds may be mirror life is very exciting. If found, it would lend tremendous evidence that life is possible in other forms as well.
Reference: "Are Aliens Among Us?" by Paul Davies, Scientific American December 2007