Generation of recombinant MP-12 encoding NSs encoding a 17 to 25 aa. truncation Using reverse genetics for the RVFV MP-12 strain, we recovered 11 recombinant MP-12 viruses which encode NSs protein with a 17 to 25 aa. truncation (Fig. 1 ). These NSs mutants exhibited different plaque phenotypes in plaque assay (Fig. S1 ) suggesting possible variation of attenuation by each NSs mutant. The plaques of NSD6-30 and NSD56-80 were clear in neutral red stain, while other mutants made turbid plaques. To test the functions of each NSs protein, VeroE6 cells (type-I IFNincompetent) were mock-infected or infected with MP-12, rMP12-C13type (a control lacking NSs functions) [18] , and NSs mutants using an moi of 3. After 16 hours, cells were collected and the abundance of PKR was measured ( Fig. 2A) as described previously [17] . As expected, PKR was not detectable in cells infected with MP-12 by post-translational downregulation [17] , but was detected in cells mock-infected or infected with rMP12-C13type. However, cells infected with the MP-12 encoding partially truncated NSs did not decrease PKR abundance.