Down syndrome (DS) is the most common genetic defect correlated with mental retardation and delayed development. The specific genes responsible for these phenotypic alterations have not yet been defined. Dyrk1A (dual-specificity tyrosine-phosphorylated and regulated kinase 1A), the human ortholog of the Drosophila minibrain gene (mnb), maps to the Down syndrome critical region of human chromosome 21 and is overexpressed in Down syndrome fetal brain. In Drosophila, minibrain is involved in postembryonic neurogenesis. In human, DYRK1A encodes a serine-threonine kinase but despite its potential involvement in the neurobiological alterations associated with Down syndrome, its physiological function has not yet been defined. To gain some insight into its biological function, we used the yeast two-hybrid approach to identify binding partners of DYRK1A. We found that the C-terminal region of DYRK1A interacts with a brain specific protein, phytanoyl-CoA alpha-hydroxylase-associated protein 1 (PAHX-AP1, also named PHYHIP) which was previously shown to interact with phytanoyl-CoA alpha-hydroxylase (PAHX, also named PHYH), a Refsum disease gene product. This interaction was confirmed by co-immunoprecipitation of PC12 cells co-transfected with DYRK1A and PAHX-AP1. Furthermore, immunofluorescence analysis of PC12 cells co-transfected with both plasmids showed a re-distribution of DYRK1A from the nucleus to the cytoplasm where it co-localized with PAHX-AP1. Finally, in PC12 cells co-transfected with both plasmids, DYRK1A was no longer able to interact with the nuclear transcription factor CREB, thereby confirming that the intracellular localization of DYRK1A was changed from the nucleus to the cytoplasm in the presence of PAHX-AP1. Therefore, these data indicate that by inducing a re-localization of DYRK1A into the cytoplasm, PAHX-AP1 may contribute to new cellular functions of DYRK1A and suggest that PAHX-AP1 may be involved in the development of neurological abnormalities observed in Down syndrome patients.