Additionally, the incidence of cGvHD was low in the HSV-TK T cell studies, and absent in the iCasp9 trial (Di Stasi et al., 2011; Zhou et al., 2014), and lymphocytes recovering from infused HSCs did not cause GVHD likely because of thymic education (Vago et al., 2012). Here, TKT is linked to graft versus host disease.