Investigations on viral gene expression during natural infection are limited by the fact that only 0.004–0.01% of mononuclear cells from seropositive granulocyte colony-stimulating factor (G-CSF)-stimulated donors carry viral genomes, with a low copy number of 2–13 genomes per infected cell, as judged by PCR-driven in situ hybridization [105]. Here, CSF3 is linked to infection.