To confirm our results from knock-down experiments, we investigated the effect of expression of the dominantnegative SD mutant and over-expression of wild-type caveolin-1 on virus production. Expression efficiency Analysis using a paired t-test (n = 9) revealed that the 1,5 to 3 fold reduction in titres compared to MDCK control cells is statistically highly significant (p > 0.001). could be monitored easily, as endogenous and transfected, recombinant caveolins differ in their mobility in SDS-PAGE due to a C-terminal myc-tag (Fig. 2 bottom) . Cav-1 appeared in two isoforms, with molecular weights of 21 and 24 kD, respectively [22, 23] . Expression efficiencies ranged from 7-29% (SD) and 20-50% (wt-Cav-1) with respect to endogenous Cav-1 level. Provided that the myc-tag does not interfere with Cav-1 antibody detection and assuming a 1:1 interaction of SD mutant and endogenous Cav-1, sufficient competitor amounts should be available in successfully transfected cells. Next, transiently transfected MDCK and mock-transfected MDCK cells were infected with influenza A/PR/8/34 virus one day after transfection. 24 h later supernatants were used for titre determination (Fig. 2 top) . To account for between-session-variations in cell culture, values were normalized to virus production from infected wt MDCK (100%). To exclude sensitivity of influenza infection to the actual transfection process, pEGFP-N1 transfected control cells were infected with PR8 virus in a control experiment. Strikingly, SD mutant expression in MDCK cells interfered with human influenza A virus replication and decreased the viral titres on average 1.6 fold to 62% of titres from wild-type MDCK (average of three independent experiments, standard deviation = ± 15,95). Compared to processed EGFP control cells, virus yield from Cav-1 wt-or SD-transfected MDCK cells was reduced considerably, which excludes that effects observed on virus production are derived from the transfection process (data not shown). This strongly suggests that interference with Cav-1 function in MDCK cells interferes with human influenza A virus replication. Interestingly, over-expression of wild-type Cav-1 also diminished influenza virus production, since viral titres reached only 56% ± 10.53 compared to non-treated MDCK (n = 3). Thus, surplus exogenous Cav-1 interferes with endogenous Cav-1 function, too. However, compared to the SD mutant twice the amount of Cav-1 wt is necessary to account for a comparable level of inhibition, as judged by Western Blot analysis.