14-3-3 proteins are highly conserved proteins which play a role in both signal transduction and progression through the cell cycle by binding to and regulating several different proteins (1.2). 14-3-3 proteins activate tyrosine and tryptophan hydroxylases and protein kinase C. They mediate signal transduction by binding to phosphoserine-containing proteins. There are at least 7 mammalian isoforms: alpha. beta. gamma. delta. epsilon. zeta. and eta. An eighth subtype. termed theta has been found in rat brain (3). The 14-3-3 proteins exists in vitro and in vivo as either homo-or heterodimers which interact via their N-terminal domains and are subject to phosphorylation by protein kinase C (4). 14-3-3 proteins are localized in the cytoplasm of neurons in the cerebral cortex and are axonally transported to the nerve terminals. They may be present at lower levels in various other eukaryotic tissues. Northern blot analysis has shown expression of the eta chain in cultured cell lines derived from various tumors (5).