We mapped 75 genes that collectively encode >90% of the proteins found in human ribosomes. Because localization of ribosomal protein genes (rp genes) is complicated by the existence of processed pseudogenes, multiple strategies were devised to identify PCR-detectable sequence-tagged sites (STSs) at introns. In some cases we exploited specific, pre-existing information about the intron/exon structure of a given human rp gene or its homolog in another vertebrate. When such information was unavailable, selection of PCR primer pairs was guided by general insights gleaned from analysis of all mammalian rp genes whose intron/exon structures have been published. For many genes, PCR amplification of introns was facilitated by use of YAC pool DNAs rather than total human genomic DNA as templates. We then assigned the rp gene STSs to individual human chromosomes by typing human-rodent hybrid cell lines. The genes were placed more precisely on the physical map of the human genome by typing of radiation hybrids or screening YAC libraries. Fifty-one previously unmapped rp genes were localized, and 24 previously reported rp gene localizations were confirmed, refined, or corrected. Though functionally related and coordinately expressed, the 75 mapped genes are widely dispersed: Both sex chromosomes and at least 20 of the 22 autosomes carry one or more rp genes. Chromosome 19, known to have a high gene density, contains an unusually large number of rp genes (12). This map provides a foundation for the study of the possible roles of ribosomal protein deficiencies in chromosomal and Mendelian disorders.[The sequence data described in this paper have been submitted to GenBank. They are listed in Table 1.]Although the ribosome, as catalyst for protein synthesis, is known to be essential for organismal growth and development, the effects of ribosomal mutations and their role in human disease have been explored barely. The mammalian ribosome is a massive structure composed of 4 RNA species and ∼80 different proteins (Wool 1979). Typical mammalian cells contain about 4 ן 10 6 ribosomes, and ribosomal RNAs and proteins constitute ∼80% of all cellular RNA and 5%-10% of cellular protein. One might predict that genetic defects in ribosomal components would invariably result in early embryonic death. However, there is strong evidence in Drosophila and suggestive evidence in humans that quantitative deficiencies of ribosomal proteins may yield viable but abnormal phenotypes. In Drosophila, the Minute phenotype (reduced body size, diminished fertility, and short, thin bristles) results from heterozygous deficiencies (deletions) at any 1 of 50 loci scattered about the genome (Schultz 1929;FlyBase 1997). Several Minute loci have been characterized molecularly, and all have been found to encode ribosomal proteins (Kongsuwan et al. 1985;Hart et al. 1993;Andersson et al. 1994;Cramton and Laski 1994;Saebøe-Larssen and Lambertsson 1996;Schmidt et al. 1996; A. Cheng, A. Zinn, J. Mach, R. Lehman, and D.C. Page, unpubl.). Thus, it appears that redu...