Diamond Blackfan Anemia (DBA) is one of several bone marrow failures that have been linked to defects in ribosome synthesis. 25% of DBA cases are linked to mutations in ribosomal protein S19 (Rps19). The etiology of the remaining cases is unknown. To gain a better understanding of the function of the Rps19 family of proteins we have characterized members of this protein family in the yeast, Saccharomyces cerevisiae. In yeast, Rps19 is encoded by duplicated genes, RPS19A and RPS19B. Yeast cells lacking both RPS19 genes are not viable, whereas those lacking a single gene are viable but have growth defects. These latter strains are defective in a specific step in rRNA processing that preferentially affects the maturation of 40S ribosomal subunits. We scanned other yeast strains with mutations in genes for 40S subunit proteins for processing phenotypes similar to RPS19 mutants. Several have phenotypes that overlap with RPS19 mutants, but only RPS18 stands out as being virtually identical to RPS19 mutants. The human RPS18 gene is therefore a candidate locus for pathogenic mutations in DBA patients with normal RPS19. We are currently developing strategies to sequence RPS18 genes from DBA patients with normal RPS19 to determine if mutations in RPS18 are associated with DBA. We have also developed a yeast system for the functional testing of mutant alleles of RPS19 found in DBA patients. In general, a mutation is considered pathogenic if it is not found in unaffected family members and in the general population. We have found, however, that several missense mutations classified as pathogenic in DBA patients do not affect Rps19 function in the yeast system. The failure of these mutations to affect Rps19 function in yeast points to a need for functional testing of RPS19 mutant alleles in human cells.