Extension of the neuronal process is a crucial step for establishment of the neuronal network. As CREB preferentially forms heterodimers with ATF1 in PC12D cells, we examined the roles of the CREB/ATF1 heterodimer on cyclic AMP (cAMP)-induced neurite extension, using originally constructed ATF1RL, which has a point mutation at the DNA binding domain of ATF1. Transient expression of ATF1RL suppressed the protein kinase A/CREB-induced expression of the CRE reporter gene as expected. Treatment with forskolin elicited a relatively poor mRNA induction for immediate early genes in PC12D-ATF1RL cells, a PC12D cell line stably expressing ATF1RL, in comparison with the parental PC12D cells. Furthermore, the PC12D-ATF1RL cells were proved to be defective at cAMP-induced neurite outgrowth. In contrast, both the gene expression and the differentiation after nerve growth factor treatment noted in PC12D-ATF1RL cells were at the same levels as those in the parental cells. These data provide us the first evidence that links CREB/ATF1 to the cAMP-induced differentiation of PC12 cells.