This project addresses a key problem in human cancer: the origins of aneuploidy. Aneuploidy is among the hallmarks of human solid tumors and is now recognized as one of the contributing factors in malignant transformation but its origin is not fully understood. While sporadic gains and losses of chromosomes can adequately explain a substantial fraction of the aberrant chromosome numbers, this explanation falls short in the context of cancers with a triploid or near-tetraploid chromosome number. It has been proposed that cancers carrying a high chromosome number originate from an unstable tetraploid intermediate. Tetraploid cells are known to mis-segregate chromosomes at a high rate, generating subclones with the sub-tetraploid or near-triploid chromosome numbers observed in cancer. These considerations have led to a quest to understand the mechanism by which tetraploidy arises during tumorigenesis. Prior to our work, three main mechanisms had been proposed. First, tetraploid cells can arise from (virally-mediated) cell-cell fusion, yielding a bi-nucleated cell which is converted into a tetraploid state during the next cell division. Second, tetraploidy can arise when cells experience a prolonged arrest in mitosis. Depending on the genetic context, mitotic slippage can occur, yielding a cell with a single tetraploid nucleus and two centrosomes in G1. Third, when the cleavage furrow is impeded, either experimentally (actin inhibition) or by a lagging chromosome, cytokinesis fails and tetraploidy arises. While each of these pathways may be operational in some cancers, they are unlikely to be a general aspect of tumorigenesis and do not explain the high frequency of tetraploidization suggested by the large percentage of tumors with a high chromosome number. Based on our preliminary data, we propose a novel mechanism for tetraploidization that involves the DNA damage signal originating from shortened dysfunctional telomeres. It has long been known that most human solid tumors experience a period of telomere shortening before telomerase is upregulated. We propose that the resulting telomere dysfunction generates a DNA damage response that can induce tetraploidization as well as other forms of genome instability, such as the Breakage-Fusion Bridge cycles resulting from dicentric chromosomes. This hypothesis is attractive because it can explain the generality and high frequency of tetraploidization in solid tumors. In addition, the insult driving tetraploidization - telomere dysfunction - is a transient state. The current project is designed to test this new hypothesis. Given the large number of tumors showing evidence of past tetraploidization, our work has the potential to have broad impact on the knowledge of the origins of aneuploidy in cancer.