4',6-diamidino-2-phenylindole or DAPI for short, is a fluorescent stain that binds to A-T regions of DNA in live or fixed cells. It was originally synthesised with the hope of becoming a drug to treat trypanosomiasis. Unfortunately, that didn't pan out. However, now it is a common nuclear stain that is used very often in fluorescence microscopy and appears fluoresces blue when bound. It is commonly used with other fluorescent stains because the blue color is quite distinguishable.