**3.6** *In vivo* **imaging**

Human retrovirus encoding *Ppy* Fluc or x11 Fluc genes bearing a myc tag was used to transduce Raji cells, which were sorted to the same expression levels (Fig. 5A) by flow cytometric sorting for myc tag staining and cultured at 37ºC. As an example of *in vivo* imaging using x11 Fluc, one million Raji cells expressing similar levels of WT or x11 Flucs were injected into the tail veins of immunocompromised Beta2m-mice to induce systemic lymphoma (Chao *et al.*, 2011) and imaged after i.p. administration of LH2. Images revealed light signals predominantly from brain, spine and hips. x11 Fluc appeared very bright *in vivo* (Fig. 5B) because of its high thermostability and pH tolerance along with favourable kinetic parameters demonstrated in characterisation. It is expected to perform equally well under the changing physiological pH conditions and in combination with aminoluciferin used for imaging of protease assays *in vivo*. Further work to codon-optimise and test this mutant in mammalian cells is underway and will be published shortly.
