DAPI

DAPI (4',6-diamidino-2-phenylindole) is a fluorescent stain that strongly binds to A-T regions of DNA and RNA. While it was originally synthesized as a candidate drug for trypanosomiasis, DAPI instead found widespread use as a blue fluorescent nuclear stain in both living and fixed cells. When bound to double-stranded DNA, the dye has an absorption maximum around 358 nm and an emission maximum around 461 nm. DAPI can also bind RNA, but with less intense fluorescence.