UASt regulatory sequences drive expression of a sfl exon that can give rise to a circular RNA (circRNA) in the context of the endogenous genomic locus, flanked upstream by 1000bp of endogenous intron sequence and downstream by the reverse complement of the 1000bp endogenous upstream intron sequence. The sfl coding sequence is tagged at the N-terminal end with Tag:FLAG. This transgene does result in overexpression of circRNA.