Chromosome instability (CIN) is the most striking feature of human cancers. However, how CIN drives tumor progression to metastasis remains elusive. Here we studied the role of chromosome content changes in generating the phenotypic dynamics that are required for metastasis. We isolated epithelial and mesenchymal clones from human carcinoma cell lines and showed that the epithelial clones were able to generate mesenchymal variants, which had the potential to further produce epithelial revertants autonomously. The successive acquisition of invasive mesenchymal and then epithelial phenotypes recapitulated the steps in tumor progression to metastasis. Importantly, the generation of mesenchymal variants from clonal epithelial populations was associated with subtle changes in chromosome content, which altered the chromosome transcriptome and influenced the expression of genes encoding intercellular junction (IJ) proteins, whereas the loss of chromosome 10p, which harbors the ZEB1 gene, was frequently detected in epithelial variants generated from mesenchymal clones. Knocking down these IJ genes in epithelial cells induced a mesenchymal phenotype, whereas knocking down the ZEB1 gene in mesenchymal cells induced an epithelial phenotype, demonstrating a causal role of chromosome content changes in phenotypic determination. Thus, our studies suggest a paradigm of tumor metastasis: primary epithelial carcinoma cells that lose chromosomes harboring IJ genes acquire an invasive mesenchymal phenotype, and subsequent chromosome content changes such as loss of 10p in disseminated mesenchymal cells generate epithelial variants, which can be selected for to generate epithelial tumors during metastatic colonization.tumor metastasis | chromosome instability | aneuploidy | clonal evolution | epithelial-mesenchymal transition (E-MT) C hromosome instability (CIN), defined by an elevated rate of chromosome missegregation and breakage, results in diverse chromosome abnormalities in tumor cell populations (1-7). Accumulating cytogenetic analyses of more than 60,000 cases of human cancer have indicated that most of the solid tumors contain chromosome aberrations, with each tumor displaying a distinct abnormal karyotype (Mitelman database: cgap.nci.nih.gov/ Chromosomes/Mitelman). In typical human cancers, one-quarter of the genome was affected by arm-level copy-number aberrations (8). Moreover, cancer genome sequencing revealed dynamic chromosome content changes during clonal evolution of the tumor cell population (9-12). However, how chromosome loss or gain drives tumor progression to metastasis remains elusive (13-17). Tumor metastasis is a multistep process, with the acquisition of an invasive mesenchymal phenotype being a crucial step for tumor dissemination, as is the reacquisition of an epithelial phenotype for metastatic colonization (18,19). Although clonal evolution theory has been well established as a general mechanism of tumor progression (16,20), how it contributes to the sequential phenotype acquisition leading to metastasis...