Interestingly, the loop A mutant, which contains a predicted loop of 7 nt, generated a siRNA duplex inside the cells that is slightly smaller than that of the shRNAs with a wild-type hairpin loop, suggesting the processing by Dicer into the stem, perhaps fulfilling the requirement of a length of 9 nt for the hairpin loop (46) . This mutant form of sh-B971 was not, however, able to trigger IFN activation.