Human neuronal cells used for the XMRV infection study: A. Human neural progenitor cells (NPCs) generated from a human fetal brain. Progenitor cells were labeled with human nestin-specific goat anti-mouse FITC (green) monoclonal antibody (Invitrogen, A10530). Nuclei were stained with DAPI. B. Progenitor-derived Astrocyte (PDA). PDAs express GFAP. Cells were stained using donkey anti-rabbit IgG-TRITC (orange) secondary antibody (Jackson ImmunoRes, 711-025-152). Nuclei are shown in blue (DAPI). C. Progenitor-derived neurons (PDN). PDNs express the neuron-specific class III β-tubulin (TUJ1) marker. Cells were stained using goat anti-mouse Alexa Fluor 647 (Red) secondary antibody (Invitrogen: A21241). Nuclei were stained with DAPI (blue).