The Saccharomyces cerevisiae genome contains 16 genes encoding full-size ABC transporters. Each comprises two nucleotide binding folds (NBF) alternating with transmembrane domains (TM). We have studied in detail three plasma membrane multidrug exporters: Pdr5p (TC3.A.1.205.1) and Snq2p (TC3.A.1.205.2) which share NBF-TM-NBF-TM topology as well as Yor1p (TC3.A.1.208.3) which exhibits the reciprocal TM-NBF-TM-NBF topology. The substrate specificity of Pdr5p, Snq2p and Yor1p are largely, but not totally, overlapping as shown by screening the growth inhibition by 349 toxic compounds of combinatorial deletants of these three ABC genes. Multiple deletion of 7 ABC genes (YOR1, SNQ2, PDR5, YCF1, PDR10, PDR11 and PDR15) and of two transcription activation factors (PDR1 and PDR3) renders the cell from 2 to 200 times more sensitive to numerous toxic coumpounds including antifungals used in agriculture or medicine. The use of the pdr1-3 activating mutation and when necessary of the PDR5 promoter in appropriate multideleted hosts allow high levels of expression of Pdr5p, Snq2p or Yor1 p. These overexpressed proteins exhibit ATPase activity in vitro and confer considerable multiple drug resistance in vivo. The latter property can be used for screening specific inhibitors of fungal and other ABC transporters.