We describe here a series of four vectors which facilitate in vitro construction of progressive deletions in cDNAs and subsequent very high expression of the mutant cDNAs in eukaryotic cells. These vectors are derived from pOEVl+(1) and thus contain: pSP65 bacterial plasmid sequences, the human CMV enhancer/promoter, the translation initiation region from the HS V thymidinc kinase gene, splicing and polyadenylation signals from the rabbit b-globin gene and the SV40 origin of replication (2). pE VRFO, pE VRF1 and pE VRF2 allow to express N terminally deleted proteins in every reading frame as fusion proteins starting with the first four amino acids of the HSV thymidine kinase. pEV3S contains stop codons in all reading frames and allows to express C terminally deleted cDNAs. Deletions can be generated by enzymatic treatment from one end of a cDNA clone (preferably cloned in a polylinker) and the deleted cDNA is then inserted either between the Smal and Xbal sites (for N-terminal deletions) or the BamHI and Smal sites (for C-terminal deletions) of the relevant vector. This cloning procedure introduces a Kpnl/Asp718 site at the deletion endpoint which can be conveniently determined by dideoxy sequencing with the following primers: