Shown above is immunostaining of a collection of thymic medullary epithelial cells (mTEC's) stained for Aire in green and a fate map RFP reporter in red. We recently described this powerful genetic tracing tool in Metzger et al. Cell Reports, 5:166, 2013. Such tools allow us to further define the key populations in the thymus that help enforce self-tolerance to the developing T cell repertoire.