T cells play a central role in the development of immune responses. Patients lacking T cells because of genetic defects such as DiGeorge or Nezelof syndromes and patients infected with the human immunodeficiency virus are highly susceptible to infections and cancers. The lack of adequate in vivo models of T cell neogenesis have hindered the development and clinical implementation of effective therapeutic modalities aimed at treating these and other clinically important maladies. Transplantation of severe combined immunodeficient (SCID) mice with human hematopoietic stem cells results in long-term engraftment and systemic reconstitution with human progenitor, B, and myeloid cells, but curiously, human T cells are rarely present in any tissue. While the implantation of SCID mice with human fetal thymus and liver (SCID-hu thy/liv mice) allows the