While a postdoctoral fellow working with Aaron Klug and Francis Crick at the MRC in the 1970s, Kornberg discovered the nucleosome as the basic protein complex packaging chromosomal DNA in the nucleus of eukaryotic cells. Chromosomal DNA is often termed "Chromatin" when it is bound to proteins in this manner, reflecting Walther Flemming's discovery that certain structures within the cell nucleus would absorb dyes and become visible under a microscope.