Intriguingly, mice overexpressing a modified Jab1/CSN5 show that the level of Jab1/CSN5 expression is a critical determinant of the proliferation and maintenance of hematopoietic progenitor cells in vivo, which may explain how the overexpression of Jab1/CSN5 contributes to tumor development [86]. The gene discussed is COPS5; the disease is neoplasm.