The invention relates to non-human transgenic animals expressing bioluminescent markers The invention provides a transgenic mouse whose genome comprises a nucleotide sequence encoding lucif erase, operably linked to murine control sequences whereby the lucif erase is expressed in all cells. Administration of a luciferin substrate to the mouse results in bioluminescence. The control sequence is the Rosa 26 promoter. This mouse is useful to monitor in situ cell growth, differentiation or proliferation