RRP1B and breast cancer: Using in vitro, genetic, and epidemiologic analyses, we have identified ribosomal RNA processing 1 homolog B (Rrp1b) as a factor that physically interacts with the metastasis modifier gene, Sipa1, modulates elements of metastasis predictive gene expression signatures, suppresses tumor progression in animal models, and is associated with progression and survival in pilot human breast cancer epidemiology cohorts.