TAT and cancer: To explore tumor-to-host communication in BrM, we used a labeling strategy that involved the transduction of cancer cells with a plasmid encoding both green fluorescent protein (GFP) and a modified mCherry protein containing a signal peptide (for secretion) followed by a lipid-permeable transactivator of transcription (TAT) domain (sLP-mCherry) (Figures 1A and S1A).