RU-521 is primarily used in scientific research as a selective inhibitor of the cyclic GMP-AMP synthase (cGAS) enzyme. This makes it a valuable tool for studying the cGAS-STING signaling pathway, which plays a critical role in innate immune responses to cytosolic DNA. By inhibiting cGAS, RU-521 helps researchers dissect the involvement of this pathway in inflammation, autoimmune diseases, and antiviral defense mechanisms. It is also explored in cellular and molecular studies to assess the therapeutic potential of modulating cGAS activity in conditions such as lupus, Aicardi-Goutières syndrome, and other interferonopathies. Due to its cell permeability and specificity, RU-521 is widely used in in vitro models to suppress aberrant immune activation and evaluate downstream signaling effects.