Like TALEs, a second bacteria-derived system known as CRISPR (clustered regularly inter-
spaced short palindromic repeats) also uses the sequence specificity of DNA to create customizable
genomic anchor points. However, in contrast to TALEs, in which DNA sequences are targeted
with repetitive protein sequences, CRISPR uses guide RNAs for this purpose (95). These guide
RNAs can be similarly customized to recognize almost any DNA sequence (96) and also bind to a
nuclease (Cas9). Thus, this system can be used to cut DNA directly, thereby enabling direct ma-
nipulation of genetic material (97). Additionally, by substituting a modified Cas9 protein without
nuclease activity, this system to be used as an anchor point for epigenetic activators or repressors
(98), as described above with DNMTs or Tet proteins.
One challenge to using these tools in vivo will be to restrict their function to certain genetically
defined cell types, especially in a brain structure that contains a heterogeneous neuronal popula-
tion. Typically, TALE- and CRISPR-based tools are expressed in mammalian cells using DNA
constructs, which often involve the use of viral vectors for more complex systems. Although this approach generally targets cells nonselectively (unless a virus with a specific tropism is used), it
can achieve robust expression in a relatively short time span and can be injected into specific brain
regions for research applications. However, viral tools that incorporate loxP sites can be used in
combination with Cre driver animal lines that express Cre recombinase only in genetically defined
cell types, thereby limiting expression of specific constructs to those cells alone. This approach
has recently been used with success to express discrete optogenetic actuators in distinct neuronal
subtypes (99, 100) to enable precise interrogation of brain circuit function.