Structures of double β-barrel domains of S. cerevisiae J-proteins Ydj1 (left; PDB: 1NLT), with yellow balls indicting Zinc ions) and Sis1 (right, PDB: 1C3G); generated using PyMOL software (http://pymol.sourceforge.net/). Examples of members of the two classes from Escherichia coli (Ec), Saccharomyces cerevisiae (Sc) and Homo sapiens (Hs), ZnBD present or absent, left and right, respectively, are given in boxes.