Answer: Protein fold
Viruses are considered to be living by some of the scientists because they carry the genetic material, participate in the reproduction and evolve by the process of natural selection but it lacks the lacks the cell structure that is required for an organism to be count as a living organism.
Now a new study that uses a protein fold as a evidence and suggests that viruses are living. These are encoded in the genomes of the viruses. These are the structural building block of the proteins, which are known to give them a complex 3-D structure. By comparing fold structures they find an evolutionary relationship between folds and organism.