Baseline CCR9 expression on CD4+ T cells in naïve, uninfected mice was reduced as a result of vitamin A deficiency in the MLN and intestinal mucosa but not in the spleen, confirming previous reports that homeostatic RA synthesis is a selective function of APCs in the GALT [7], [8], [10] (Figure 4). The gene discussed is CD4; the disease is vitamin A deficiency.