Seven different, but highly conserved 14-3-3 proteins are involved in diverse signaling pathways in human cells. It is unclear how the 14-3-3sigma isoform, a transcriptional target of p53, exerts its inhibitory effect on the cell cycle in the presence of other 14-3-3 isoforms, which are constitutively expressed at high levels. In order to identify structural differences between the 14-3-3 isoforms, we solved the crystal structure of the human 14-3-3sigma protein at a resolution of 2.8 Angstroms and compared it to the known structures of 14-3-3zeta and 14-3-3tau. The global architecture of the 14-3-3sigma fold is similar to the previously determined structures of 14-3-3zeta and 14-3-3t: two 14-3-3sigma molecules form a cup-shaped dimer. Significant differences between these 14-3-3 isoforms were detected adjacent to the amphipathic groove, which mediates the binding to phosphorylated consensus motifs in 14-3-3-ligands. Another specificity determining region is localized between amino-acids 203 to 215. These differences presumably select for the interaction with specific ligands, which may explain the different biological functions of the respective 14-3-3 isoforms. Furthermore, the two 14-3-3sigma molecules forming a dimer differ by the spatial position of the ninth helix, which is shifted to the inside of the ligand interaction surface, thus indicating adaptability of this part of the molecule. In addition, 5 non-conserved residues are located at the interface between two 14-3-3sigma proteins forming a dimer and represent candidate determinants of homo- and hetero-dimerization specificity. The structural differences among the 14-3-3 isoforms described here presumably contribute to isoform-specific interactions and functions.