Instead, it has been proposed that memory T cells already residing in peripheral tissues at the time of infection—either permanently as in the case of TRM cells, or temporarily as proposed for CD4+ memory T cells—are key to rapid infection control in barrier tissues such as skin and mucosa (Woodland and Kohlmeier, 2009; Di Meglio et al., 2011; Kupper, 2012; Masopust and Picker, 2012). The gene discussed is CD4; the disease is infection.