Since the E H244A mutation disrupts E protein's interaction with pr, we used this mutation to address the importance of pr in protecting DENV during transport through the exocytic pathway. We introduced the E H244A mutation into the infectious clone of DENV1 WP. WT and mutant viral RNAs were prepared by in vitro transcription and were electroporated into BHK cells. After culture for 3 d at 37uC, both WT and mutant RNA-electroporated cells expressed abundant E protein as detected by immunofluorescence microscopy (Fig. 7) . Parallel cultures were incubated for 6 d and progeny virus in the culture media was detected by infectious center assays on indicator BHK cells. WTinfected cells produced infectious progeny virus with a titer of ,1.5610 5 IC/ml. However, two independent infectious clones of the H244A mutant produced no detectable progeny virus, even though the viral RNAs mediated efficient primary infection as shown in Fig. 7 . This agrees with previous studies indicating lethal effects of an H244A mutation on DENV2 [43] .