As breast tumors progress, the matrix surrounding the tumor cells becomes stiffer. How normal cells sense and respond to the changes in stiffness was not well understood. Now Lily Thao-Nhi Le and colleagues in ASCB member Lindsay Hinck’s lab at University of California, Santa Cruz found that during breast morphogenesis, signaling protein Robo1 is important for actin stress fiber assembly and cell contractility. They also found that a stiff matrix increases signaling protein Robo1 by down regulating micro-RNA miR-203. Patients with high Robo1 and low miR-203 show better prognosis and survival. Published in the Journal of Cell Biology.