Axonal regeneration after crush injury of the sciatic nerve has been intensely studied for the elucidation of molecular and cellular mechanisms. Neurite extension factor1 (Nrsn 1) is a unique membranous protein that has a microtubule-binding domain and is specifically expressed in neurons. Our studies have shown that Nrsn I is localized particularly in actively extending neurites, thus playing a role in membrane transport to the growing distal ends of extending neurites. To elucidate the possible role of Nrsn I during peripheral axonal regeneration, we examined the expression of Nrsn I mRNA by in situ hybridization and Nrsn I localization by immunocytochemistry, using a mouse model. The results revealed that during the early phase of axonal regeneration of motor nerves, Nrsn I mRNA is upregulated in the injured motor neuron. Nrsn I is localized in the cell bodies of motor neurons and at the growing distal ends of regenerating axons. These results indicate that Nrsn I plays an active role in axonal regeneration as well as in embryonic development. (c) 2007 Elsevier Ireland Ltd. All rights reserved.