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A flexible cytoskeleton is a good attribute for a stem cell, but it means bad news for a mature cell. As cells differentiate they become stiffer, and if a cell regains the ability to change shape easily, it also gains metastatic potential. Zhang et al. used this increase in flexibility to separate metastatic from nonmetastatic cells. As cells navigate a maze of microbarriers in a microfluidic chip, stiff cells get stuck, whereas flexible cells make it through and are collected at the end of the maze. The researchers ran metastatic and nonmetastatic breast cancer cells through their chip to prove the principle and then used the device to enrich for flexible cells in a heterogeneous population of breast cancer cells. The deformable cells overexpress genes related to cell motility and metastasis and show anchorage-independent growth.