pWALEXA20 allows cloning of short hairpins as annealed synthetic oligos, targeted against specific genes via the production of short hairpin microRNAs; contains white as a selectable marker; 13 copies of lexAop2 (arranged in three groups, as a pentamer, trimer and pentamer); the hsp70 basal promoter; a multiple cloning site (MCS) for cloning shRNAs in a mir-1 scaffold; a ftz 3'UTR intron followed by a SV40 3'UTR to facilitate shRNA processing and export from the nucleus; two gypsy insulator sequences flanking the transcription unit to enhance hairpin DNA transcription; a single loxP site (upstream of the lexAop2 sequences); an attB sequence to allow phiC31:int-targeted integration at genomic attP landing sites.