Superresolution methods are breakthrough in science that permits imaging well below the diffraction limit of light (＜200-nanometer).Many superresolution methods are utilizing reversibly photoswitchable fluorescent proteins (RSFPs) whose fluorescent status is controlled either manually or stochastically.Most of RSFPs are categorized in negatively switchable RSFP (NS-RSFP), which share the light for switching-off with that for fluorescence excitation [1].Their applications are limited because light illumination for fluorescence excitation induces switching-off of the NS-RSFP by which fluorescence signal to be acquired should be reduced.On the other hand, positively photoswitchable RSFP (PS-RSFP) can be switched on by a wavelength for fluorescence excitation, escaping the above problem in NS-RSFP.However, the slow switching speed, low brightness and weak photostability in conventional PS-RSFPs have restricted their applications to superresolution imaging.To overcome this, we describe Kohinoor, a fast-switching, positively photoswitchable fluorescent protein, and show that it has good photostability over many switching repeats (Tiwari DK et al.Nature Methods, 2015).With Kohinoor, we achieved super-resolution imaging of live HeLa cells using biocompatible, ultralow laser intensity (0.004 J/cm2) in reversible saturable optical fluorescence transition (RESOLFT) nanoscopy.