Retention of fluorescently labeled affibodies (black, WT; blue, N23BP; red, Z) in either (A) transfected 4T1 cells induced with 15 μg/mL cumate or (B) MDA-MB-468 cells, each grown as an adherent monolayer of cells. Wells were either irradiated after 3.5 h of incubation for 30 min with 365 nm light (dashed lines, triangles) or kept in the dark (solid lines, circles). Concentrations of retained affibodies were calculated by comparing spheroid lysate fluorescence to standard curves prepared for each labeled affibody.