The gene coding for a major, nonribosomal protein of the nucleolus, nucleolin, has been isolated and characterized in three rodent species (Bourbon et al., 1988a and b). A sequence comparison of the 5′ terminal regions indicated a remarkable conservation within around 800bp upstream from the start sites and extending about 825bp downstream into the first intron. It was determined that this highly conserved region constituted an extended CpG island (Bird, 1987), whose boundaries and G+C content are likewise conserved in the three species (Bourbon et al., 1988b). In an attempt to define DNA elements of the 5′ terminal regions which are potentially active in the transcriptional regulation of the gene, we made a compilation of the conserved sequences and identified stretches, termed homology blocks, in which 7 out of 8 nt had been conserved among the species. This evolution-oriented approach to define motifs important for promoter function provided us with a series of putative cis-acting regulatory elements. In this work, we have tested the effectiveness of the 5′ upstream region which contains these conserved motifs to act as a promoter using a functional assay and we have determined the relative promotion strength of the region.