For instance, induction of HLA-DRantigens on human promyelocytic leukaemia HL-60 cells byinterleukin-4 (IL-4) is of pivotal importance in immunology notonly because class II expression is prerequisite to antigenrecognition and response but also because IL-4 participates in aplethora of inflammatory or non-inflammatory reactions. This evidence concerns the gene IL4 and acute promyelocytic leukemia.