Down regulation of ubiquitin gene expression during the differentiation of human leukemia cells (20), over expression of CDC34 (21), UbcH5A and UbcH6 (22), E2N (CCRF-CEM) in acute lymphoblastic leukemia (23), induction of UBCH8 in leukemic patients treated with chemotherapy (24) and differential expression of UBE2N and UBE2C (also called UbcH10) between different types of lymphoma cell lines and hematological malignancies (25-27) are among the numerous examples of the involvement of ubiquitin conjugating enzymes in leukemia development. This evidence concerns the gene UBE2E1 and acute lymphoblastic leukemia.