A wild-type haploid yeast strain was transformed with a library of wild-type yeast DNA fragments ligated into a high-copy-number plasmid vector (YEp24). The pooled URA+ transformants were plated on rich medium containing a lethal concentration of trifluoperazine (TFP) (4,26). The 530-base-pair (bp) PvuII-ClaI fragment (Fig. 1) was used as the hybridization probe for the TFP3 locus. The probe for chromosome XIII was the ILV2 gene, provided by S. Carl Falco (10).An insertion and disruption construction was made by digesting pTFP3-40 at the unique PvuII site at amino acid 24