Dansyl chloride is a yellow-orange crystalline chemical which cannot be dissolved in water but can be dissolved in acetone (a type of ketone), and has the chemical formula C12H12ClNO2S. It will melt at temperatures between 66.5 and 68 degrees Celsius. "Dansyl" is an acronym for 5 dimethylamino-1 naphthalenesulfonyl.
Dansyl chloride is used as a marker in molecular biology experiments. It is fluorescent when it is attached to a protein or polypeptide, and is used to identify the amino acid which is on the amino terminal end of a polypeptide chain.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.