These models indicated a role for T cells, particularly CD4+, in both the mediation of CLL engraftment (Bagnara et al., 2011; Devereux, 2011; Dürig et al., 2007; Shimoni et al., 1999) and the resultant CLL cell disappearance with concomitant evidence of lethal graft-versus-host disease (GvHD) (Bagnara et al., 2011; Devereux, 2011). This evidence concerns the gene CD4 and graft versus host disease.