2',5'-Oligoadenylate (2-5A)-dependent RNase L is a ubiquitous endoribonuclease of higher vertebrates that functions in the interferon (IFN) antiviral response by degrading both viral and cellular single-stranded RNA (ssRNA). In addition, the RNase L gene, RNASEL, was mapped to the hereditary prostate cancer 1 (HPC1) gene. Previous analyses of human RNASEL determined its exon/intron structure but lacked a description of the promoter region. We thus mapped the RNASEL transcriptional start site using 5'-rapid amplification of cDNA ends (5'-RACE) and primer extension methods with RNA from human histiocytic lymphoma U937 cells. The promoter sequence was analyzed for potential transcription factor binding sites. Although a canonical IFN-gamma activation site (GAS) element (TTCCAAGAA) was identified (nucleotides -155 to -147), there was only slight induction of RNASEL promoter-reporter activity or of endogenous RNase L expression in response to IFN-alpha or IFN-gamma. Several sites for tissue-specific and general promoters were observed, however, which could explain the widespread expression of RNase L in mammalian cells. Accordingly, RNase L levels were determined and compared in different human and rodent cancer and normal cell types using a radiolabeled 2-5A derivative. In addition, levels of RNase L were established in various normal human tissues and cell types by immunoblotting and immunohistochemistry. Our findings are the first description of the human RNASEL promoter that allows constituitive expression in a range of normal and neoplastic cell types.