Our understanding of the cancer metastatic process has advanced considerably in recent years. However, the early stages of tumor progression and micrometastasis formation have been difficult to analyze. These studies are hampered by the inability to identify small numbers of tumor cells against a background of many host cells. The visualization of tumor cell emboli, and micrometastases and their progression over real time during the course of the disease has been difficult to study in current models of metastasis. Previous studies used transfection of tumor cells with the Escherichia coli β-galactosi-dase (lac Z) gene to detect micrometastases (1 ,2 ,33 ). However, detection of lac Z requires extensive histological preparation, and therefore it is impossible to detect and visualize tumor cells in viable fresh tissue or the live animal at the microscopic level. The visualization of tumor invasion and micrometastasis formation in viable fresh tissue or the live animal is necessary for a critical understanding of tumor progression and its control.