Scribble is a cell polarity protein that localizes to cell-cell junctions and cell membranes. Loss of Scribble expression is known to function as a tumor suppressor in multiple organs. However, Scribble is frequently amplified and mislocalized in multiple carcinoma including breast, prostate, lung, and head and neck. To begin to understand the effect of mislocalizing Scribble, we generated a transgenic mouse model expressing a mislocalizing mutant, Prol305Leu, under the control of the MMTV promoter. In the transgenic mouse mammary gland, SCRIB protein is expressed in the cytosol (green) of luminal epithelial cells and not in the CK14 positive (red) basal epithelia. These mice develop spontaneous tumors after a long latency, demonstrating that mislocalization of Scribble is sufficient to initiate tumorigenesis. For details, see article by Feigin and colleagues on page 3180.