Phosphorylation of Y221 in the Picchu-X sensor was triggered by EGFR activation following EGF stimulation (Figures 1a and c); the associated increase in FRET efficiency of the biosensor in transfected HCC1954 breast cancer cells is shown in Figure 1d (shift to ‘warmer’ colours in pseudocoloured micrographs) and Figure 1e. Here, EGFR is linked to breast carcinoma.