P-element mutagenesis using the SynP1+P2 strain containing the P{lacW}SynP1 and P{lacW}SynP2 insertions has resulted in precise excision of the P{lacW}SynP2 insertion together with an imprecise excision of the P{lacW}SynP1 insertion. The imprecise excision results in the deletion of 10bp around the original insertion site.