The intrinsic brightness of fluorescent proteins has been taken advantage of to develop a technology of whole-body imaging of tumors and gene expression in mouse internal organs. Stable transformation with fluorescent protein genes can be effected using retroviral vectors containing a selectable marker such as neomycin resistance. The cells that stably express fluorescent proteins can then be transplanted into appropriate mouse models. For whole-body imaging, nude mice are very appropriate. If wild-type mice are used, then hair must be removed by shaving or depilation. The instruments used can range from a simple LED flashlight and appropriate excitation and emission filters to sophisticated equipment such as the Olympus OV100 with a wide range of magnification, enabling both macroimaging and microimaging. It is crucial that proper filters be used such that background autofluorescence is minimal. Fluorescent protein-based imaging technology can be used for whole-body imaging of fluorescent cells on essentially all organs. The timeline for these experiments varies from 2 days to 2 months.