UNKNOWN GENE SOURCE. “In general terms, the claim that giant viruses represent a fourth domain of life simply refers to the cell-like character of these viruses in terms of size of the virions and genomes and, in addition, to the observation that many genes of these viruses have no detectable homologs and so could have come from some unknown source.”
http://onlinelibrary.wiley.com/doi/10.1111/nyas.12728/full