Encodes a sna minigene expressed under the control of its own promoter. Sequence encoding a poly-glycine linker followed by a VHH(GBP1) nanobody has been inserted at the C-terminal end of the open reading frame. 24 copies of Tag:MS2 have been introduced (flanked by hb intron sequence) at the 5' end of the coding sequence (this enables nascent mRNA to be detected).