The mechanisms of virus-insect vector interactions are categorized as three major transmission modes, non-circulative, circulative-propagative and circulative-non-propagative. The circulative non-propagative transmission has solely been reported for plant viruses. In this mode of transmission, the virus mandatorily cycles within its insect vector, traversing cellular barriers of the gut and salivary glands, without replicating. Virus particles are believed to pass across vector cells through transcytosis, sequestered into membranous vesicles and thus not in direct contact with the cyctoplasm or nucleoplasm. However, this has been experimentally shown for the family Luteoviridae only, and the mechanisms of cell crossing for the families Gemini- and Nanoviridae remain hypothetical. In the two latter families, empirical observations are sometimes questioning the absence of viral replication in the insect cells, denoting that this mode of transmission requires increased research efforts for precisely defining the intimate relationship between viruses and their vectors. We have initiated a study on the molecular/cellular interactions between aphids and the nanovirus species Faba bean necrotic stunt virus (FBNSV). We have established the route of this multipartite virus in the aphid organs and tissues, where distinct viral genome segments all travel together. Both other and our groups have shown the requirement of a “helper” factor for aphid-transmission of FBNSV. This helper factor is being identified as the product of the genome segment N, the protein NSP. At this point, we have evidence that the NSP protein controls the entry and/or accumulation of FBNSV into the aphid gut cells and further study of its mode of action is in progress.