In addition to monoclonal immunoglobulin. two kinds of nucleoproteins. NP1 and NP2, were isolated from the supernatants of hybridoma cultures set up in a protein-free medium. As shown by SDS-electrophoresis the two nucleoproteins shared a set or proteins (apparent M r 11000 to 15000). and differed in the DNA moiety (∼ 150 bp in NP1. ∼ 350 bp in NP2). The amino acid composition of the protein moiety confirmed the nucleosomal origin of NP1 and NP2. The findings support the view that in hybridoma cultures the cells undergo death by apoptosis, i.e. a programmed process characterized by initial fragmentation of chromatin.
