Structures of ZHX HD domains. (A) ZHX1 HD4 showing the two molecules in the crystal asymmetric unit (B) ZHX2 HD2 showing the two molecules in the crystal asymmetric unit. The disulfide bridge linking the subunits is shown in light blue and yellow (C) Part of ZHX1 HD4 showing omit electron density for residues 458-462, a region where the switch in helix I conformation occurs in ZHX2 HD2 compared to ZHX1 HD4. The monomer A is coloured in orange and monomer B in cyan. The dotted lines represent hydrogen bonds. The CA trace of a standard HD conformation is shown as thin black sticks.