SP 600125, as a negative control for JNK Inhibitor II, is primarily used in research settings to validate the specificity and efficacy of JNK pathway inhibition studies. It serves as a comparative baseline in experiments to ensure that observed effects are due to JNK inhibition and not other non-specific interactions. Researchers employ it in cell-based assays and in vivo models to study the role of the JNK signaling pathway in processes like apoptosis, inflammation, and stress responses. Its application helps in distinguishing true JNK-mediated effects from off-target or background activities, ensuring accurate interpretation of experimental results.