A sca2::Tn mutant strain of R. rickettsii was attenuated following intradermal (i.d.)infection in a guinea pig model (15), and a sca2::Tn mutant strain of R. parkeri was attenuated following i.d. infection of mice carrying mutations in the genes encoding the receptors for IFN-I (Ifnar1) and IFN-γ (Ifngr1) (Ifnar1−/−; Ifngr1−/− double-knockout [DKO] mice [17]), suggesting that Sca2 is a virulence factor. This evidence concerns the gene IFNG and infection.