The discrepancy regarding PRR4 as a marker in humanized mice versus human patients may result from the fact that the hematopoietic system is the only source of human PRR4 in humanized mice, while in human patients PRR4 from other sources, such as the salivary and lacrimal glands, may obscure PTLD-specific patterns [35]. The gene discussed is PRR4; the disease is post-transplant lymphoproliferative disease.