N-Cas9(5)::FRB and FKBP::C-Cas9(5) form the two halves of a rapamycin-inducible split RNA-guided nuclease system that is based on Cas9. N-Cas9(5)::FRB consists of an N-terminal fragment of Cas9 (residues 2-573), tagged at the N-terminal end with a nuclear export signal (NES) and at the C-terminal end with Tag:FRB. FKBP::C-Cas9(5) consists of a C-terminal fragment of Cas9 (residues 574-1368), tagged at the N-terminal end with both a nuclear localization signal (NLS) and Tag:FKBP and at the C-terminal end with a second NLS. In the absence of rapamycin, the two split portions of Cas9 are separated into the nuclear and cytoplasmic compartments (due to the presence of the NES in N-Cas9(5)::FRB and the NLS sequences in FKBP::C-Cas9(5)). In the presence of rapamycin, heterodimerization of Tag:FRB and Tag:FKBP promotes the association of N-Cas9(5)::FRB and (newly synthesized) FKBP::C-Cas9(5) proteins, reconstituting a functional Cas9 protein that is translocated to the nucleus due to the presence of the NLS sequences in FKBP::C-Cas9(5) (PMID:25643054, FBrf0245398).