In order to identify the organ site of viral uptake and IFN-ab production in vivo, we analyzed the expression of the early viral gene E1A and of type I IFN mRNAs, respectively, in the spleen, liver, lung and kidney of Ad3-infected mice. We found expression of the early viral E1A gene in spleen and liver (Fig. S1B ), but not in lung and kidney (not shown), which agrees with earlier findings on in vivo Ad tropism [47, 48] . Expression of IFN-a and IFN-b mRNA was below the level of detection in the organs of non-infected controls. We also found that between 4 and 18 h after infection IFN-a and IFN-b mRNAs were expressed at high levels in the spleen (Fig. 1F ), but surprisingly not in the liver, despite the presence of Ad in both organs. As expected, IFN-ab was not expressed in the virus-free lung or kidney of infected mice (Fig. 1F) .