T regulatory cells (also known as Tregs or Regulatory T cells) are essential cells in\nthe immune system that suppress immune responses of other cells, designed to\nlimit excessive reactions and prevent autoimmunity. Tregs are characterized by the\nexpression of CD4, CD25, and Foxp3, while lacking CD127. CD4+Foxp3+ regulatory\nT cells have been referred to as \u201cnaturally-occurring\u201d regulatory T cells to distinguish\nthem from \u201csuppressor\u201d T cell populations that are generated in vitro. While other\nvariants of suppressive T cells do exist, such as CD8 suppressor cells, Th3 and Tr-1 cells,\nTregs are classically defined as CD4+CD25+Foxp3+ cells. Here, we provide a protocol\nfor immunofluorescent staining of FOXP3 in frozen sections, adapted from a protocol\nfrom Dr. Matthias Hardtke-Wolenski, Medizinische Hochschule Hannover. This protocol\nhas been successfully used on mouse spleen, liver, and skin sections.