In several previous investigations caveolin-1 (Cav-1), a multifunctional, raft-resident membrane protein has been linked to the virus replication of retroviruses HIV-1 and amphotropic mouse leukemia virus, rotavirus and respiratory syncytial virus [9] [10] [11] [12] [13] . Interestingly, a contribution of Cav-1 to HA transport has been reported for influenza virus infected MDCK cells [14] . In a recent investigation of the enveloped γ-retroviruses budding from lipid rafts we showed that caveolin-1 (Cav-1) interacts specifically with the MLV retroviral matrix protein in the Gag precursor, suggesting that Cav-1 serves in positioning the Gag precursor at lipid rafts [13] . Not surprisingly, Cav-1 is incorporated into MLV virions released from mouse NIH3T3 [13, 15] . Subsequently, competition and inhibition experiments provided evidence that Cav-1 modulates MLV retrovirus production [13] . Taken together, these findings pointed to a general contribution of Cav-1 in virus replication strategy and opened the possibility that other virus families budding from lipid rafts may co-opt the functions of Cav-1. In our search for cellular/viral targets a database screen for Cav-1 binding sites notably revealed that structural proteins like matrix proteins of other viral families, e.g. Orthomyxoviridae with influenza A virus as a representative, exhibit regions of homology with a consensus motif for Cav-1 binding (Cav-1 binding domain, CBD) (Wirth, M, unpublished) .