In mouse mammary epithelial cells, cytoplasmic polyadenylation element binding protein 1 (CPEB1) mediates the apical localization of ZO-1 mRNA, which encodes a critical tight junction component. In mice lacking CPEB1 and in cultured cells from which CPEB has been depleted, randomly distributed ZO-1 mRNA leads to the loss of cell polarity. We have investigated whether this diminution of polarity results in an epithelial-to-mesenchyme (EMT) transition and possible increased metastatic potential. Here, we show that CPEB1-depleted mammary epithelial cells alter their gene expression profile in a manner consistent with an EMT and also become motile, which are made particularly robust when cells are treated with transforming growth factor-β, an enhancer of EMT. CPEB1-depleted mammary cells become metastatic to the lung following injection into mouse fat pads while ectopically expressed CPEB1 prevents metastasis. Surprisingly, CPEB1 depletion causes some EMT/metastasis-related mRNAs to have shorter poly(A) tails while other mRNAs to have longer poly(A) tails. Matrix metalloproteinase 9 (MMP9) mRNA, which encodes a metastasis-promoting factor, undergoes poly(A) lengthening and enhanced translation upon CPEB reduction. Moreover, in human breast cancer cells that become progressively more metastatic, CPEB1 is reduced while MMP9 becomes more abundant. These data suggest that at least in part, CPEB1 regulation of MMP9 mRNA expression mediates metastasis of breast cancer cells.