The 18S rRNA is highly conserved among eukaryotes. We have several configurations of assays for this gene: P/N 4331182, Assay ID: Hs99999901_s1; PN 4319413E, 4310893E, 4333760T, 4333760F, 4308329. You can use any of these for both human and mouse samples, as well as samples from any other eukaryotic species.