Practice question

Cre-Off or double floxed inverted open reading frame systems express gene constitutively before recombination. Coding sequence flanked by incompatible lox sites in forward orientation remains active under promoter. Cre mediated inversion using opposite orientation loxP or lox2272 flips gene into antisense orientation, often followed by excision locking orientation, extinguishing transcription or producing non-functional transcript. Therefore activity before Cre allows labeling or function, after Cre signal terminates. This inversion dependent off switch enables Cre mediated silencing in specific neurons for circuit mapping.