A. The crystal structure of the BH3-in-groove Bax homodimer (PDB entry 4BDU) is shown with one monomer colored gray and other colored green, magenta, blue and red for its α2, α3, α4 and α5 helices, respectively, as indicated. The residue pairs that were replaced with cysteine pairs in (B) are presented in stick form, and their β carbon atoms linked by dashed lines with the distances ranging from 5.0 to 6.0 Å. 