In the presence of salicylic acid, NahR activated transcription from Psal and thus produced XylS2, which then subsequently bound the effector molecule salicylic acid, becoming activated and causing synergistically increased transcription from Pm. The system was demonstrated to be useful for studying bacterium–host interactions in vivo in both mouse and tumour cells by expressing the GFP protein from the Pm promoter (for a review, see Becker et al., 2010). Here, PRB1 is linked to neoplasm.