pyCTZ (Pyridyl CTZ) hydrochloride, a pyridyl Coelenterazine (CTZ) analog, and is an ATP-independent pyridyl substrate of LumiLuc luciferase. pyCTZ hydrochloride generates strong blue bioluminescence in the presence of luciferases. pyCTZ hydrochloride can be used for aequorin-based calcium sensing.