<p class="first" id="P1">The CRISPR (clustered regularly interspaced short palindromic
repeat)-Cas system is
an adaptive immune system of bacteria that has furnished several RNA-guided DNA endonucleases
(e.g., Cas9) that are revolutionizing the field of genome engineering. Cas9 is being
used to effect genomic alterations as well as in gene drives, where a particular trait
may be propagated through a targeted species population over several generations.
The ease of targeting catalytically impaired Cas9 to any genomic loci has led to development
of technologies for base editing, chromatin imaging and modeling, epigenetic editing,
and gene regulation. Unsurprisingly, Cas9 is being developed for numerous applications
in biotechnology and biomedical research and as a gene therapy agent for multiple
pathologies. There is a need for precise control of Cas9 activity over several dimensions,
including those of dose, time, and space in these applications. Such precision controls,
which are required of therapeutic agents, are particularly important for Cas9 as off-target
effects, chromosomal translocations, immunogenic response, genotoxicity, and embryonic
mosaicism are observed at elevated levels and with prolonged activity of Cas9. Here,
we provide a perspective on advances in the precision control of Cas9 over aforementioned
dimensions using external stimuli (e.g., small molecules or light) for controlled
activation, inhibition, or degradation of Cas9.
</p><p id="P2">
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