Expression of the transcription factor FOXC2 is induced and necessary for successful epithelial-mesenchymal transition, a developmental program that when activated in cancer endows cells with metastatic potential and the properties of stem cells. As such, identifying agents that inhibit the growth of FOXC2-transformed cells represents an attractive approach to inhibit chemotherapy resistance and metastatic dissemination. From a high throughput synthetic lethal screen, we identified a small molecule, FiVe1, which selectively and irreversibly inhibits the growth of mesenchymally transformed breast cancer cells and soft tissue sarcomas of diverse histological subtypes. FiVe1 targets the intermediate filament and mesenchymal marker vimentin (VIM) in a mode which promotes VIM disorganization and phosphorylation during metaphase, ultimately leading to mitotic catastrophe, multinucleation, and the loss of stemness. These findings illustrate a previously undescribed mechanism for interrupting faithful mitotic progression and may ultimately inform the design of therapies for a broad range of mesenchymal cancers.vimentin | epithelial-to-mesenchymal transition | cancer stem cell | mitosis | drug discovery F or many tumor types, resistance to conventional chemotherapy and subsequent tumor relapse have been attributed to the presence of a slower cycling, drug-resistant population of cells termed cancer stem cells (CSCs) or tumor initiating cells. We and others have demonstrated that activation of a latent embryonic program, called the epithelial-mesenchymal transition (EMT), endows epithelial-derived cancer cells with the properties of stem cells as well as migratory and metastatic potential (1). Cells undergoing EMT exhibit the loss of epithelial cell-cell contacts (e.g., E-cadherin), the induction of matrix degrading proteases (e.g., matrix metalloproteinases), and the acquisition of motility-inducing intermediate filaments [e.g., vimentin (VIM)], features which promote metastatic progression by allowing dissemination from the local tumor niche (2). A number of transcription factors (e.g., Snail, Twist, ZEB1) and microenvironment-derived extracellular signaling molecules (e.g., TGF-β1) are capable of inducing the EMT transcriptional program. Among these factors, we have demonstrated that the transcription factor Forkhead Box C2 (FOXC2) is a central regulator of EMT in breast cancer (3, 4). FOXC2 expression is both up-regulated and required for the induction of CSC properties by the classical EMT-inducing factors Twist, Snail, and TGF-β1 (4