Close-up view of three zinc finger structures. A The first Zn1 ion (red) is engaged by residues C20, C51, H54, and H56. B The second Zn2 ion (blue) is engaged by residues C143, C146, C161, and C164. C The third Zn3 ion (yellow) is engaged by residues C190, C193, H202, and C236. All the residues are the same color (C, orange; N, blue; S, yellow; O, red). D The topology of these three zinc fingers. Blue cylinders and red arrows represent helices and β-sheets, respectively.