Insertion of a P{PTT-GC} splice trap element into the large first intron of mdu, approximately 10.6kb downstream of the splice site in exon 1. The element is inserted in the correct orientation to trap transcripts of mdu, which would result in an N-terminal GFP tag on an almost undisrupted protein (exon 1 encodes only 7 amino acids, including the initiator Met codon).