For a reliable distinction, however, further parameters, e.g., cell growth or cell age, as well as cell adhesion to a substrate23 should be considered. In addition, other types of cancer cells should be examined by similar methods. For example, MCF-7 breast cancer cells transfected either with the proliferation inhibitor and apoptosis regulator p21 or with the pro-oncogene c-myc showed only minor differences of fluorescence spectra and lifetimes in comparison with MCF-7 control cells (results not shown). Finally, in tissue further fluorescent components, resulting e.g., from extracellular collagen or elastic fibers,7,24,25 as well as an inhomogeneous illumination, resulting in variations of fluorescence intensity, should be taken into account. Due to all these uncertainties it is suggested to combine autofluorescence measurements with other label-free methods, e.g., Raman scattering.26–28 Preliminary Raman experiments29 exhibited slight differences between U251-MG control cells and cells with activated suppressor genes, mainly originating from granules surrounding the cell nucleus. However, only future experiments will prove whether these differences can be quantified reliably.