Mimivirus, short for "mimicking microbe", is so called to reflect its large size and apparent Gram-staining properties.

In 2003, researchers at the Université de la Méditerranée in Marseille, France, published a paper in Science identifying the micro-organism as a virus.

[12] It does not appear to possess an outer viral envelope, suggesting that the virus does not exit the host cell by exocytosis.

The condensed central core of the virion appears as a dark region under the electron microscope.

However, three distinct aminoacyl tRNA synthetase enzyme transcripts and four unknown mRNA molecules specific to mimivirus were also found.

[13] The mimivirus genome is a linear, double-stranded molecule of DNA with 1,181,404 base pairs in length.

Once inside, an eclipse phase begins, in which the virus disappears and all appears normal within the cell.

In addition, mimivirus has genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack.

[citation needed][21] Because its lineage is very old and could have emerged prior to cellular organisms,[22][23] Mimivirus has added to the debate over the origins of life.

This has been used to suggest that Mimivirus is related to a type of DNA virus that emerged before cellular organisms and played a key role in the development of all life on Earth.

[22] An alternative hypothesis is that there were three distinct types of DNA viruses that were involved in generating the three known domains of life—eukarya, archaea and bacteria.

[21] Nevertheless, mimivirus does not exhibit the following characteristics, all of which are part of many conventional definitions of life:[citation needed]