PRKDC and severe combined immunodeficiency: The Prkdcscid (protein kinase, DNA activated, catalytic polypeptide) mutation in CB17 mice, commonly denoted “SCID” (severe combined immunodeficiency) (197), results in reduced numbers of functional T and B cells and thus enables limited and transient engraftment of human PBMCs, HSCs and fetal hematopoietic tissues (198–200).