Multicolored proteins have allowed the color-coding of cancer cells growing in vivo and enabled the distinction of host from tumor with single-cell resolution. Non-invasive imaging with fluorescent proteins enabled the dynamics of metastatic cancer to be followed in real time in individual animals. Non-invasive imaging of cancer cells expressing fluorescent proteins has allowed the real-time determination of efficacy of candidate antitumor and antimetastatic agents in mouse models. The use of fluorescent proteins to differentially label cancer cells in the nucleus and cytoplasm can visualize the nuclear-cytoplasmic dynamics of cancer cells in vivo including: mitosis, apoptosis, cell-cycle position, and differential behavior of nucleus and cytoplasm that occurs during cancer-cell deformation and extravasation. Recent applications of the technology described here include linking fluorescent proteins with cell-cycle-specific proteins such that the cells change color from red to green as they transit from G1 to S phases. With the macro- and micro-imaging technologies described here, essentially any in vivo process can be imaged, giving rise to the new field of in vivo cell biology using fluorescent proteins.