XBP1 and infection: GRP78, the major indicator of ER stress can be induced by XBP1(s) or ATF6, was up-regulated early at 3 h.p.i. (data not shown), with a peak at 6 h.p.i. and a gradual reduction to basal levels at later stages of infection (24 h.p.i, Figure 1B), strongly suggesting that the IRE1-XBP1 signal is responsible for the up-regulation of GRP78.