1. Any of the substances present in the nucleus of a cell, consisting chiefly of proteins, phosphoric acids, and nucleic acids.
2. A decomposition product of nucleoprotein intermediate between native nucleoprotein and nucleic acid.

It is a colorless, amorphous compound, soluble in dilute alkalis, but insoluble in dilute acids. The nucleins consist of nucleic acid and bases which vary in the different nucleins.