Combined treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) and dibutyryl cyclic AMP (diBu-cAMP) induced significantly longer neurites than treatment with each alone in NG108-15 cells. We performed differential screening to identify genes expressed only by treatment with TPA plus diBu-cAMP but not by that with diBu-cAMP for 72 h alone in NG108-15 cells, and isolated a novel gene, TA20. Over-expression of the gene in NG108-15 and neuroblastoma N18TG-2 cells caused intense neurite elongation and suppressed cell growth. TA20 did not cause, however, any morphological changes in glioma C6Bu-1 cells. These results suggest that TA20 is a novel neuronal differentiation factor.