BTK and fibrosarcoma: Figure 3 and Supplementary Fig. S2 summarize some of the results confirming target binding, specificity via blocking, and the ability to perform live cell imaging. To quantify co-localization between the imaging probe and BTK at the subcellular level, we created a stable transgenic cell line expressing a BTK-mCherry fusion protein in HT1080 human fibrosarcoma cells. In vitro cell experiments showed excellent co-localization and blocking (r2 = 0.9851; Fig. 4).