Cre-FLEX system allows for:
Engineered Cre dependent vectors utilize double floxed inverted orientation or FLEX switch. Wild type and mutant loxP pairs are arranged antiparallel flanking gene in inverted orientation. In absence of Cre, transcription proceeds in wrong orientation or cassette blocked. Upon Cre expression, sequential recombination events invert coding sequence into correct orientation then excise two lox sites locking it permanently active or inactive. This enables irreversible flipping. Such reversible flipping before locking allows tight Cre dependent control for optogenetics, chemogenetics and cell type specific gene expression manipulation.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.