重链抗体不结合轻链，其可变区单独就可以结合抗原，体外表达以后大小为几个纳米级别，所以又称纳米抗体。近几年来，纳米抗体在实验室研究、小分子检测、临床诊断以及药物研发等方面展现出了极强的应用价值。但天然情况下纳米抗体只能从驼科动物中获得，这在一定程度上限制了其应用。牛抗体重链基因位点中具有超长的D片段，通过V（D）J重组，可以使牛抗体形成超长的CDR3。这些CDR3内部的多个半胱氨酸通过二硫键使其形成稳定的特殊结构，从而使CDR3单独就可以决定抗原结合特异性。基于牛免疫球蛋白的这种特性，本研究拟尝试探索回答两个问题：1，利用抗原免疫牛后，是否可以筛选获得具有抗原特异性的纳米抗体？2，将超长的牛D基因片段定点整合入小鼠免疫球蛋白基因位点，小鼠是否可以表达具有超长CDR3的抗体？这些小鼠是否可以用来制备抗原特异性的纳米抗体？本研究旨在拓展纳米抗体制备的动物来源，推动纳米抗体的广泛应用。

Without light chains, heavy chain only antibodies (HCAbs) can bind antigens with their own variable regions. When expressed in vitro, the size of the HCAb variable regions appears to be approximately 2.5-4 nm, the HCAb variable regions are therefore called nano-antibodies. Although nano-antibodies have been widely applied in many areas such as laboratory researches, monitoring of small molecule chemicals, clinical diagnosis and development of new antibody drugs, naturally, they can only be derived from camelids, which, to some extent, limits a better application of nano-antibodies. Compared with other species, the cattle is characterized with exceptionally long DH gene segments (the longest one: 149 bp), which obviously contribute to the exceptionally long CDR3 observed in the bovine immunoglobulin heavy chains. Cysteines within the CDR3 are able to stabilize the CDR3 structure by forming disulfide bonds, thus allowing the CDR3 itself bind to antigens directly. Based on the unique character of the bovine immunoglobulin heavy chain genes, we aim to address two questions: 1, Can the cattle be used to generate antigen specific nano-antibodies when immunized? 2, Can the mice express antibodies with exceptionally long CDR3 if the bovine long DH gene segments were precisely integrated into the endogenous immunoglobulin heavy chain gene locus? Can these mice be used generate antigen specific nano-antibodies when immunized? This study aims to establish new animal systems for production of nano-antibodies, which would facilitate the application of nano-antibodies by complementing the camelid antibodies.