Ramon y Cajal University Hospital 1 article published in JoVE Biology Stimulation of Stem Cell Niches and Tissue Regeneration in Mouse Skin by Switchable Protoporphyrin IX-Dependent Photogeneration of Reactive Oxygen Species In Situ Jesús Espada1,2, Elisa Carrasco3, María I. Calvo-Sánchez1,4, Sandra Fernández-Martos1, Juan José Montoya5 1Ramón y Cajal Institute for Health Research (IRYCIS), Ramón y Cajal University Hospital, 2 The aim of this protocol is to induce transient in vivo production of nonlethal levels of reactive oxygen species (ROS) in mouse skin, further promoting physiological responses in the tissue.