(A) Cultured cancer cells were exposed to FITC derivatives of BIM BH3 peptides (1 μM) for 2 hours, followed by centrifugation, trypsinization, washing, lysate preparation, electrophoresis, and fluorescence detection. FITC-BIM SAHBA and its R153D point mutant were present at similar levels in the cellular lysates, whereas no uptake was observed for the unmodified FITC-BIM BH3 peptide. (B) Live cell confocal microscopy of U937 cancer cells treated with FITC-BIM SAHBAs (1 μM, 2 hours) demonstrated the uptake and intracellular distribution of the peptides. FITC-BIM SAHBA, but not the impaired R153D mutant, largely colocalizes with mitochondria, the site of BCL-2 family targets. Nuclei (Hoechst), SAHBs (FITC), mitochondria (MitoTracker), and the BIM SAHBA/mitochondria colocalization (FITC/MitoTracker) are shown in blue, green, red, and yellow, respectively. An example of the BIM SAHBA/mitochondria colocalization is highlighted by the white arrow. Original magnification, ×100.