We next tested the ability of each of the eight Fab-catalysts tolabel HER2 and neighbors on a well-characterized breast cancer-derivedcell line, SKBR3, where HER2 is overexpressed (1.6 million receptorsper cell), leading to constitutive self-activation.36 Each of the eight Fab-catalysts were added, at 5 μg/mL(120 nM), to 5 million SKBR3 cells for 30 min at 4 °C.Excess Fab was quickly washed away followed by labeling with 100 μMdiazirine–biotin for 10 min of blue light exposure. Here, ERBB2 is linked to breast cancer.