The authors constructed a series of recombinant genomes that express chloramphenicol acetyltransferase (CAT) in mammalian cells. The prototype, pSV2-cat, consists of the beta-lactamase gene and origin of replication from pBR322 coupled to a simian virus 40 (SV40) early transcription region. CAT activity was readily measured within 48 hours after introducing pSV2-cat DNA into African green monkey kidney CV-1 cells. The absence of endogenous CAT activity in these cells, combined with sensitive assays for CAT activity, made this system ideal for monitoring foreign DNA expression in tissue culture. The authors also constructed derivatives of pSV2-cat by removing parts or all of the SV40 promoter region to demonstrate the system's utility. Deletion of one copy of the 72-base-pair repeat sequence in the SV40 promoter had no significant effect on CAT synthesis, while deleting an additional 50 base pairs reduced CAT synthesis to 11% of the wild type. Another recombinant, pSV0-cat, was constructed with the entire SV40 promoter region removed and a unique HindIII site for easy insertion of other promoter sequences. The study highlights the advantages of using CAT as an assay for promoter function in mammalian cells, including the absence of interfering endogenous activities and the rapid, sensitive, and reproducible nature of the assay.The authors constructed a series of recombinant genomes that express chloramphenicol acetyltransferase (CAT) in mammalian cells. The prototype, pSV2-cat, consists of the beta-lactamase gene and origin of replication from pBR322 coupled to a simian virus 40 (SV40) early transcription region. CAT activity was readily measured within 48 hours after introducing pSV2-cat DNA into African green monkey kidney CV-1 cells. The absence of endogenous CAT activity in these cells, combined with sensitive assays for CAT activity, made this system ideal for monitoring foreign DNA expression in tissue culture. The authors also constructed derivatives of pSV2-cat by removing parts or all of the SV40 promoter region to demonstrate the system's utility. Deletion of one copy of the 72-base-pair repeat sequence in the SV40 promoter had no significant effect on CAT synthesis, while deleting an additional 50 base pairs reduced CAT synthesis to 11% of the wild type. Another recombinant, pSV0-cat, was constructed with the entire SV40 promoter region removed and a unique HindIII site for easy insertion of other promoter sequences. The study highlights the advantages of using CAT as an assay for promoter function in mammalian cells, including the absence of interfering endogenous activities and the rapid, sensitive, and reproducible nature of the assay.