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From The Collaborative International Dictionary of English v.0.48:
Nuclein \Nu"cle*in\, n. (Physiol. Chem.) A constituent of the nuclei of all cells, containing protein and nucleic acid. It is a colorless amorphous substance, readily soluble in alkaline fluids and especially characterized by its comparatively large content of phosphorus. It also contains nitrogen and sulphur. [1913 Webster +PJC]