The CRISPR-Cas9 genome-editing system is a part of the adaptive immune system in archaea and bacteria to defend against invasive nucleic acids from phages and plasmids. The single guide RNA (sgRNA) of the system recognizes its target sequence in the genome, and the Cas9 nuclease of the system acts as a pair of scissors to cleave the double strands of DNA. Since its discovery, CRISPR-Cas9 has become the most robust platform for genome engineering in eukaryotic cells. Recently, the CRISPR-Cas9 system has triggered enormous interest in therapeutic applications. CRISPR-Cas9 can be applied to correct disease-causing gene mutations or engineer T cells for cancer immunotherapy. The first clinical trial using the CRISPR-Cas9 technology was conducted in 2016. Despite the great promise of the CRISPR-Cas9 technology, several challenges remain to be tackled before its successful applications for human patients. The greatest challenge is the safe and efficient delivery of the CRISPR-Cas9 genome-editing system to target cells in human body. In this review, we will introduce the molecular mechanism and different strategies to edit genes using the CRISPR-Cas9 system. We will then highlight the current systems that have been developed to deliver CRISPR-Cas9 in vitro and in vivo for various therapeutic purposes.

CRISPR-Cas9基因组编辑系统是古细菌和细菌中适应性免疫系统的一部分，可以防御噬菌体和质粒的入侵核酸。该系统的单向导RNA（sgRNA）识别其在基因组中的靶序列，该系统的Cas9核酸酶充当剪刀来切割DNA的双链。自发现以来，CRISPR-Cas9已成为真核细胞基因组工程最强大的平台。最近，CRISPR-Cas9系统引发了对治疗应用的极大兴趣。CRISPR-Cas9可用于纠正引起疾病的基因突变或改造T细胞用于癌症免疫治疗。使用CRISPR-Cas9技术的首次临床试验于2016年进行。尽管CRISPR-Cas9技术的前景广阔，在将其成功应用于人类患者之前，仍需要解决一些挑战。最大的挑战是将CRISPR-Cas9基因组编辑系统安全有效地递送至人体靶细胞。在这篇综述中，我们将介绍使用CRISPR-Cas9系统编辑基因的分子机制和不同策略。然后我们将重点介绍为提供CRISPR-Cas9而开发的当前系统体外和体内用于各种治疗目的。