ZM 39923 HCl is primarily utilized in research settings as a selective inhibitor of Janus kinase 3 (JAK3). It plays a significant role in studying signal transduction pathways, particularly those involved in immune responses. Researchers use it to investigate the effects of JAK3 inhibition on T-cell activation and proliferation, which is crucial for understanding autoimmune diseases and developing targeted therapies. Additionally, it aids in exploring the role of JAK3 in hematopoiesis and its potential implications in treating certain cancers. Its application extends to preclinical studies aimed at developing novel immunosuppressive drugs.