We chose the murine melanoma cell line 4599, which was derived from tumours arising in the BRAFV600E mouse model53 and shown to spontaneously metastasise when transplanted intradermally.15 To render 4599 cells traceable in vivo, we engineered them using lentiviral technology to constitutively express the radionuclide reporter NIS fused C-terminally to the red fluorescent protein mCherry (Fig. 3a). Here, SLC5A5 is linked to melanoma.