To dissect whether alopecia and dermatitis are manifestations of the same disease or involve different mechanisms, we firstly bred the Rosa26fl/fl-Yfp mice with Ox40-Cre mice to generate Rosa26fl/fl-Yfp–Ox40-Cre mice, in which cells that have ever-expressed OX40 (past and present) are genetically marked by the yellow fluorescent protein (YFP). The gene discussed is TNFRSF4; the disease is dermatitis.