The P{(FRT.w⁺)Wee-P.ph1} element contains an artificial exon that consists of a splice donor site, the EGFP coding sequence (with its ATG codon and a consensus translation initiation sequence just 5' to the ATG) and a splice acceptor site. The exon is designed to act as a protein trap element when inserted into a phase 1 coding intron of an endogenous gene in the correct orientation. The presence of the translation start sequence means that insertions into the 5'UTR of an endogenous gene may act as a promoter trap, also resulting in EGFP expression. The element also contains an excisable FRT cassette that contains the w^+mW.hs marker.