Figure 2.

Two major approaches to access cell types. A. Utilizing gene expression regulatory mechanisms. Toolgene (“effector”) can be inserted into host genome via genetic engineering, or delivered into cells on a viral vector or plasmids. Its expression can be regulated on the DNA level by a binary system such as Cre-loxP(①), on the transcriptional level promoter and enhancers(②), on the post-transcription level by miRNAs (③), or on the protein level through modulating protein stability or localization(④). B. Recognizing cell surface molecules. Viral infection is mediated by interaction between capsid protein and membrane receptors (⑤). Nanobody recognizing cell surface antigen, or engineered ligands recognizing membrane receptors, can be used to decorate virus or non-viral vehicles such as nanoparticles which modulates neuron activity or delivers drugs (⑥).