UPF1 and bacterial infectious disease: As seen in Fig. 3a,b, a transient induction in UPF1 phosphorylation, peaking at 2.5 to 3 times the signal observed for unstimulated cells, occurred on treatment of RAW 264.7 macrophages with lipopolysaccharides (LPS, mimicking bacterial infection; Fig. 3a) and of NIH 3T3 fibroblasts with serum (mimicking injury; Fig. 3b, upper panel).