5-Methylindolo[3,2-b]quinoline (cryptolepine) is primarily used in pharmaceutical research, especially for developing anti-cancer drugs. It inhibits cancer cell division at the molecular level by interfering with topoisomerase enzymes, which are essential for DNA replication in cancer cells. It also has potential as a fluorescent agent in bioimaging and cellular studies, exhibiting fluorescence upon binding to DNA, which allows for membrane penetration and detection in biological environments. Additionally, its strong fluorescence and electron-donating properties make it a building block for fluorescent dyes and optoelectronic materials, with applications in photovoltaic devices and OLED dopants.