BI-78D3 is a small molecule inhibitor primarily used in research settings to target and inhibit the RSK (p90 ribosomal S6 kinase) family of proteins, especially RSK1 and RSK2. It is valued for its role in blocking signaling pathways involved in cell proliferation and survival, particularly those downstream of the Ras/MAPK pathway. This makes it a useful tool in studying cancers driven by mutations in this pathway, such as certain types of melanoma and breast cancer. Its application extends to cellular and molecular studies where it helps elucidate the role of RSK in processes like transcription regulation, cell motility, and tumor progression. Due to its ability to induce apoptosis and reduce cell growth in RSK-dependent cancer models, BI-78D3 serves as a prototype for developing targeted cancer therapies. However, it remains primarily a research compound and is not used clinically.