In other words, retrograde infection of V2 FB neurons by only three of the vectors (CAV2-Cre, AAV-Flex-TVA-mCherry, and RVdG-GFP) is sufficient to double-label these cells, but infection by all four vectors (the three above plus AAV-Flex-oG) must occur for the double-labeled cells to act as starter cells. Here, CAV2 is linked to infection.