The homeodomain protein Arix/Phox2a plays a role in the development and maintenance of the noradrenergic cell type by regulating the transcription of genes involved in the biosynthesis and metabolism of noradrenaline. Previous work has shown that Arix/Phox2a is a phosphoprotein, and the phosphorylated form of Arix/Phox2a exhibits poorer DNA-binding activity than does the dephosphorylated form. Here, we demonstrate that Arix/Phox2a is phosphorylated by extracellular signal-related kinase (ERK)1/2 at two sites within the N-terminal transactivation domain. The phosphorylation level of Arix in cultured SH-SY5Y neuroblastoma cells is reduced when cells are treated with the mitogen activated protein kinase kinase 1 (MEK1) inhibitor UO126. Treatment of sympathetic neurons with the MEK1 inhibitor, PD98059, results in an elevation of mRNAs encoding noradrenergic proteins, dopamine beta-hydroxylase (DBH) and norepinephrine transporter (NET), but not tyrosine hydroyxlase (TH). Treatment of neuroblastoma cultures with PD98059 increases the interaction of Arix with DBH and NET genes, but not the TH gene. Together, these results suggest that phosphorylation of Arix by ERK1/2 inhibits its ability to interact with target genes, and that both specificity of expression and modulation by external stimuli are monitored through the same transcription factor.