Résumé : Previous work on mouse x rat hybrid cells (BS series) led to the assignment of a transformation suppressor locus (Sail) to the rat 5q22-q33 region. This gene is not yet identified. From a non-transformed BS hybrid cell line, we isolated a partial cDNA insert (13T), which detects a transcript more abundant in transformed cells than in their non-transformed homologs. Sequence comparisons led us to conclude that 13T is identical to the coding sequences of the ribosomal protein S3a gene (Rps-3α), of Fte-1 (v-fos transformation effector gene) and of TU-11, a mouse gene induced by TNF-α. Rps3α, Fte-1 and TU-II are thus one and the same gene. Similarity was also found between this gene,and non-mammalian sequences reported to be involved in cell cycling. Like the Rps3α transcript level, the c-Fos transcript level is higher in transformed cells. Rps3α and Fos could thus be effecters of the transformed phenotype.