Principle of Brainbow strategies. In Brainbow-1.0 and Brainbow-1.1, pairs of incompatible (heterospecific) lox variants are interleaved, creating two or three mutually exclusive excision possibilities. Each of these excisions triggers the expression of a distinct XFP. In Brainbow-2.0, loxP sites are positioned in opposite orientation, defining an invertible DNA segment in which two XFP genes are placed head to head. This construct can invert as long as Cre is active. When it stabilizes, the XFP gene ending in a sense orientation is expressed. In Brainbow-2.1, two invertible units are positioned in tandem, offering additional excision and inversion options, and yielding a total of four expression possibilities.