Hoechst 33342 trihydrochloride (Bisbenzimide ethoxide trihydrochloride, HOE 33342 trihydrochloride) is a bis-benzimide fluorescent dye that specifically binds to AT-rich sequences in the minor groove of double-stranded DNA. When exposed to UV light, it emits bright fluorescence at Hoechst Red (630–650 nm) and Hoechst Blue (405–450 nm) wavelengths. This dye is used to determine cell cycle status, conduct apoptosis assays, and act as a nuclear marker in microscopy.