J. Donner, S. Thompson and his co-workers from the ICFO-Institut de Ciencies Fotoniques in Barcelona, Spain have devised a way to use GFP to map intracellular temperature in cells. Their method is based on the temperature dependence of fluorescence polarization anisotropy of GFP. According to their paper published in Nano Letters, “When the temperature increases, the Brownian rotational motion of the fluorophores is accelerated. Hence, the molecules will rotate more during their fluorescence lifetime. The more the molecules rotate during their fluorescence lifetime, the more the re-emitted photons will lose the memory of the incident light polarization. Consequently, a temperature increase leads to a decrease of the degree of polarization (anisotropy) of the fluorescence.” The method was tested and worked well in HeLa and U-87 MG cancer cells.