Bcl-2-deficient mice (Veis et al., 1993) and mice lacking myeloid cell leukemia 1 (Mcl-1) in the hematopoietic system (Opferman et al., 2005) exhibit profound lymphopenia, whereas mice overexpressing a Bcl-2 transgene throughout the hematopoietic compartment show marked lymphadenopathy (Ogilvy et al., 1999). Here, MCL1 is linked to lymphopenia.