ZM39923 is primarily used in research settings as a potent and selective inhibitor of JAK3 (Janus kinase 3). Its application is significant in studying the role of JAK3 in immune cell signaling and its involvement in autoimmune diseases, inflammatory conditions, and cancer. Researchers utilize ZM39923 to explore potential therapeutic strategies for disorders like rheumatoid arthritis, psoriasis, lupus, transplant rejection, and hematological malignancies such as leukemia, where JAK3 signaling plays a critical role. By inhibiting JAK3, ZM39923 helps in understanding the pathways and mechanisms underlying these diseases, paving the way for the development of targeted treatments. Additionally, it serves as a valuable tool in biochemical assays to dissect the specific functions of JAK3 in cellular processes.