Human ribosomal protein S6 kinase includes two protein families: P90RSK and P70S6K, they participate in two different signaling pathways. When the two kinases were inhibited by their antibodies or rapamycin, the proliferation of cells was arrested. However, their analog, the immunosupressant FK-506, can inhibit the proliferation of fibroblast PBL1 without interfering with the activities of P90RSK, P70S6K and MAPK. We take the tactics of "homolog screening" to demonstrate whether there are some novel proteins which can substitute for the known P90RSK and P70S6K or other pathways without interfering with the known P90RSK and P70S6K. With the conserved sequence of mouse p90RSK as a probe, we screened the homologous sequence in NCBI EST database and got three human EST fragments. With the assembled contig as a probe to screen human brain cDNA library, a full-length cDNA of 3833 bp was attained. It contains a completed open reading frame from 165 bp to 2570 bp encoding 802 amino acids. The putative protein has higher homology with other members of p90RSK family. The gene was named RPS6KA5, the accession number in GenBank is AF090421. Northern hybridization showed the gene expressed in 16 human tissues tested, and the gene was localized in 14q31-32.1 by RH mapping. Another novel P70S6K gene has also been cloned. Thus, our initial presumption that there is an analog of known P90RSK and P70S6K in human beings was proved.