The P{Wee-P.ph0} element is composed essentially of 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 0 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. A single FRT site is present 5' to the protein trap cassette.