The major histocompatibility (MHC) class Ib molecules HLA-E, HLA-F and HLA-G are relatively non-polymorphic compared to class Ia molecules. Both HLA-E and HLA-G bind peptides and are involved in natural killer (NK)-cell recognition, but the role of HLA-F is unclear. HLA-E binds specifically to the conserved leader sequence peptides from the class Ia MHC molecules and interacts on the cell surface with the CD94/NKG2 class of NK-cell receptors. The framework structure of HLA-E is similar to that of the MHC class Ia molecules, but the peptide-binding groove is highly adapted for the specific binding of the leader sequence peptides. This is different from class Ia molecules, which have highly promiscuous peptide-binding grooves. The HLA-E groove makes full use of all the available pockets and imposes specificity along the entire length of the peptide. HLA-G binds nonamer peptides with leucine or isoleucine at position 2, proline at position 3 and leucine at position 9. Expression of HLA-G inhibits NK cells expressing the CD94/NKG2 class of receptors, though an interaction with these receptors has not been directly demonstrated.