Increased percentages of Ki-67-expressing T cells have been observed in different clinical conditions of lymphopenia, including in HIV infection, after SCT, and post-thymectomy (Hazenberg et al., 2000b, 2002; Borghans et al., 2006; van Gent et al., 2011), and also under more physiological circumstances in aging rhesus macaques and humans (Naylor et al., 2005; Cicin-Sain et al., 2007; Sauce et al., 2012). This evidence concerns the gene MKI67 and lymphopenia.