How endogenously tag your desired gene
Tagging a gene endogenously was previously challenging and had to be done in more crude ways such as using cDNA fusion proteins which can be delivered transiently (AAV) or constitutively (Lenti-virus). However, expression of these constructs is not under control of an endogenous promoter and wont temporally recapitulate the protein. With CRISPR/Cas9, endogenously tagging is now possible due to creating double stranded breaks (DSB) nearly anywhere in the genome in combination with a donor DNA template.
The first decision is what side of the gene you want to tag (start or stop codon) (Figure 1), which might be dependent on: