A 300bp fragment ("PRE300") containing an active portion of a pairing-sensitive element near eve (fragment extends from +8.9kb to +9.2kb relative to the eve transcription start site) has been inserted at an XbaI site in a reporter containing a bxd enhancer fragment inserted upstream of a Ubx fragment driving lacZ expression. The PRE300 fragment is oriented such that the 3' end is closer to the Ubx promoter.