Schematic of three possible interference mechanisms mediated by Fv1 expression. Although the mechanism of Fv1 interference is still poorly understood and firm experimental evidence is lacking, several likely routes can be envisaged. Route A depicts the binding of Fv1 to CA, thereby restricting CA participation in the integration of the pre-integration complex (PIC) of MuLV DNA. In favour of this model, crystallographic studies recently suggested that a potential Fv1 binding domain exists in the MuLV CA . Alternatively, if yet undetermined CA helper factors (CAhf) are needed during CA mediated integration of the PIC; binding of Fv1 to CAhf would prevent CAhf to assist CA during integration of the PIC (route B). A third possible route would involve direct binding of Fv1 to the PIC, thereby changing its conformation, and restricting it from further processing during CA-mediated integration (route C).