NKG mice are a type of severe immunodeficient mouse developed by Cyagen by deleting the Il2rg gene from the NOD-Scid strain. This strain lacks mature T, B, and NK cells, exhibits reduced complement activity, and weak macrophage phagocytosis of human cells. As a result, NKG mice can efficiently engraft human hematopoietic stem cells (HSC), peripheral blood mononuclear cells (PBMC), patient-derived xenografts (PDX), or adult stem cells and tissues.

Neonatal Fc receptor (FcRn) is a cell surface receptor protein that binds to the Fc region of IgG antibodies. It is structurally similar to MHC class I molecules and comprises an α-chain and β2-microglobulin (β2M). The α-chain of the FcRn receptor is encoded by the Fcγ receptor and transporter (FCGRT) gene, while β2-microglobulin is encoded by the β-2-microglobulin (B2M) gene. FcRn is expressed widely on epithelial cells, endothelial cells, and hematopoietic cells, and is found in various tissues and organs, including the intestine, placenta, kidney, and liver ^[1-2].

IgG antibodies are the most abundant immunoglobulins in human serum (about 75%), and play an important role in the immune response by defending against pathogens and toxins. Compared to other immunoglobulins, IgG has a high circulating level, a longer half-life, and the ability to be transferred from mother to offspring. These properties are closely related to its interaction with FcRn. FcRn binds to the Fc region of IgG, preventing IgG molecules from being degraded by lysosomes. This prolongs the in vivo half-life of IgG and is involved in the transport, maintenance, and distribution metabolism of IgG. In addition, the specific transport process of IgG from the mother to the fetus to provide the fetus with short-term passive immunity is also mediated by FcRn ^[1-2]. In addition to its protective role, IgG autoantibodies are also associated with many pathological conditions. Therefore, novel FcRn blocking therapies are an effective strategy to reduce the circulating levels of pathogenic IgG autoantibodies and to reduce IgG-mediated diseases. In addition, many drugs also utilize FcRn's protective mechanism for IgG by fusing or conjugating with the Fc portion of IgG to prolong its serum half-life and improve its pharmacokinetics. The FCGRT gene encodes the α-chain of the FcRn protein, and its homologous genes are present in most mammals.

The NKG-hFcRn mouse is a humanized model constructed by replacing the exon 2 coding region plus partial intron 2 of the mouse Fcgrt gene in situ with the "Human FCGRT CDS-3'UTR of Mouse Fcgrt-WPRE-BGH pA" cassette. It expresses human FcRn to replace the endogenous mouse FcRn. The homozygous NKG-hFcRn mice are viable and fertile. This model may help evaluate the pharmacokinetics/pharmacodynamics of human immunoglobulin G in transplantation research, human tumor xenografts, and mouse tumor allografts.