The mammalian ras genes have been implicated in a wide variety of natural and experimental tumors. They code for small GTP binding proteins which are believed to play a central role in the control of cellular proliferation and differentiation. We have investigated the transcriptional organization of the human N-ras locus and characterized a transcription unit located immediately upstream of N-ras, which we designate by NRU for N-ras Upstream. NRU messages contain an open reading frame of 767 amino acids which shows no similarity with the ras proteins and provides no clue to the function of the corresponding protein. Of the order of 150 nucleotides separate the N-ras transcription initiation sites from the last NRU polyadenylation site and the same organization is present in the murine genome. Both genes are simultaneously expressed in all the cell lines and murine tissues we have analysed, and in all cases NRU messages accumulate to a higher level than those of N-ras. The small intergenic distance implies that, during transcription of NRU, RNA polymerases transcribe a substantial part of the N-ras gene, suggesting that this genetic organization participates in the regulation of N-ras expression.