The major histocompatibility complex (MHC) is a cluster of genes that are important in the immune response to infections. In humans, this complex is referred to as the human leukocyte antigen (HLA) region. HLA G belongs to the HLA class I molecules (HLA Class Ib; nonclassical) and is involved in the presentation of foreign antigens to the immune system. This molecule is expressed on the surface of trophoblast cells and plays a role in maternal tolerance of the fetus by mediating protection from the deleterious effects of natural killer cells, cytotoxic T lymphocytes, macrophages and mononuclear cells.