The molecular machinery governing cell polarity controls various aspects of cell behaviors including directional migration of cells under monolayer culture conditions. Immunofluorescence revealed that the Par-1 kinase, a critical component of the polarity-regulating machinery, is enriched on the lateral side of such migrating cells. However, in cells that were surrounded by extracellular matrix proteins, including tumor cells that have recently extravasated into the parenchyma of foreign tissues during metastasis, the Par-1 kinase was diffusely distributed throughout the cytosol. In these cells, cytosolically distributed Par-1 interfered with the critical signaling pathway driving cell proliferation, accounting for the difficulty of the recently extravasated tumor cells to initiate rapid proliferation. For details, see the article by Shibue and colleagues on page 5944.