We used nucleic acid hybridization and cDNA cloning techniques to isolate human sequences that respond to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). These clones were used as probes to examine changes of gene expression that occurred after the proliferation of exponentially growing primary human fibroblasts was arrested. Transcript levels detected by these probes were increased coordinately by treatment of the cells with UV light, mitomycin C, TPA, or the UV light-induced extracellular protein synthesis-inducing factor EPIF (M. Schorpp, U. Mallick, H. J. Rahmsdorf, and P. Herrlich, Cell 37:861-868, 1984). Proteins coded for by these transcripts were characterized by hybrid-promoted translation and by cDNA sequencing. One of the cDNA clones was homologous to the metallothionein IIa gene, and one set of related clones selected RNA for the secreted TPA-inducible protein XHF1 (U. Mallick, H. J. Rahmsdorf, N. Yamamoto, H. Ponta, R.-D. Wegner, and P. Herrlich, Proc. Natl. Acad. Sci. USA 79:7886-7890, 1982).