Cell competition can occur between distinct genotypes within multicellular organs. We mapped a recessive mutation that prevents competition of RpL36+/- cells to the gene encoding Ribosomal Protein S12. Null alleles of RpS12 are recessive lethal, and we have generated several transgenic fly lines and showed that null alleles can be rescued by genomic constructs lacking the major introns and their encoded snoRNAs, confirming that it is the protein coding region that is essential. We have also produced RpS12 specific antibodies and demonstrated their specificity by immunoprecipitation and immunostaining assays. We have described the localization of the RpS12 protein in wild type and Minute genotypes, and in the suppressor allele for cell competition. Immunostaining of Drosophila salivary glands and wing imaginal discs showed that RpS12 is mainly distributed in the cytoplasm. A substantial amount of RpS12 is also detected around the nucleolus. There is no clear difference for the cellular localization among these three genotypes. Future work will be focused on mapping the region of RpS12 protein that is important in cell competition and its mechanism of action.