High Id4 expression is observed in bladder[14] and rat mammary gland carcinomas,[15], whereas chromosomal translocation of Id4 (t(6;14)(p22;q32)) was found in B-cell acute lymphoblastic leukemia[16] and B-cell precursor acute lymphoblastic leukemia (BCP-ALL)[17], suggesting that it may also have tumor promoter activity. Here, ID4 is linked to B-cell acute lymphoblastic leukemia.