TI{20XUAS-lexA::p65-OR-FLP} is designed to generate mosaic tissues composed of clones which express either lexA::p65 or FLP and has the following structure: 20xUAS regulatory sequences - attB site - lexA::p65 sequence in inverted orientation relative to UAS - attP site - STOP sequence - attP site - FLP sequence. In the absence of phiC31:int recombinase neither lexA::p65 nor FLP is expressed. In the presence of phiC31:int, recombination can occur between the attB site and one of the two attP sites in a stochastic manner, with no further recombination possible. If recombination occurs between the attB site and the attP site upstream of the STOP sequence, the lexA::p65 sequence is inverted, putting it in the correct orientation for expression under the control of the UAS sequences (FLP is still not expressed due to the STOP sequence). If recombination instead occurs between the attB site and the attP site downstream of the STOP sequence, lexA::p65 and the STOP sequence are excised, allowing expression of FLP. The first recombination event (resulting in lexA::p65 expression) occurs at a higher frequency than the second recombination event.