Fig. 1.

Overview of Cas endonuclease activity and nucleic acid detection systems. a Cas endonucleases are single protein effectors for guide RNAs. b Cas endonucleases bind targets complementary to target DNA (Cas9 or Cas12) or RNA (Cas13) and cleave the target (Cas9 or Cas12). c For Cas12a and Cas13, target binding also activates collateral cleavage of single-stranded DNA (ssDNA; Cas12a) or single-stranded RNA (ssRNA; Cas13) supplied in trans. d This activity can be exploited for nucleic acid detection. For attomolar sensitivity, isothermal amplification of input nucleic acid is required. The resulting DNA can be transcribed (txn) for Cas13-based detection, or detected directly by Cas12a. Reporter ssRNA or ssDNA are cleaved by Cas13 or Cas12a, respectively, producing a fluorescent signal. dsDNA double-stranded DNA