CRISPR: off-target concern

1. Ranked causes

   1. Guide has high predicted off-target sites (poor specificity score)Design tools flag multiple high-scoring off-targets in the genome used.
   2. High Cas / guide dose driving promiscuous cleavageLowering RNP dose preserves on-target while reducing off-target PCR hits.
   3. Cell-line genetic background differs from the reference used for designOff-target amplicon matches an alternate allele present in this line.
   4. Phenotype from clonal variation / stress, not an off-target editOrthogonal genotyping of top off-targets is clean; independent clones differ.

2. Recovery steps

   1. Re-design with a higher-specificity guide or high-fidelity Cas variantAddresses poor specificity at the design step.
   2. Titrate RNP down while monitoring on-target efficiencyDose-driven promiscuity is a common reversible cause.
   3. Sequence top predicted off-targets (and authenticate the line)Separates true off-targets from clonal noise or wrong-line genetics.

3. Controls first

   • Did unedited / mock-transfected controls show the same phenotype?If yes: Culture stress or assay — not necessarily an off-target edit.
   • Do top predicted off-target amplicons show indels?If yes: Treat as a real off-target risk — retarget or use a high-fidelity nuclease.

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