Insertional biases. A) Number of transposable element insertions in introns: introns of canonical GFP-tagged genes were divided into 'trap' if they carried the GFP-trap insertion and 'non-trap' if not. Targets were independently analysed for P-element and piggyBac insertions. While the 'trapped' introns generally show large numbers of previously reported P-element or piggyBac insertions, the 'non-trapped' introns show significantly lower values. B) Intron length: genes with previous P-element or piggyBac insertions were divided into 'trapped' or 'non-trapped' depending on the existence of a GFP-trap insertion. The average length of the introns hit by P-element traps is 9.2 kb and is 18.2 kb for piggyBac targets, which is significantly higher than the 1.9 kb and 1.8 kb average for other non-trapped introns in the same genes. C) Expression duration of genes: embryonic gene expression was divided into three roughly equivalent time frames. GFP-trap targets show prolonged gene expression compared to other genes susceptible to P-element-insertion (data for piggyBac not shown). The small number of genes with no embryonic expression originated from the pilot screen performed on L1 larvae.