Insertion of a gene trap construct results in a fusion protein that would retain the first 139 amino acids including the conserved amino terminus. The fusion protein is predicted to lack the last 986 amino acids of the wild-type protein, including the highly conserved proline-rich domain, the microtubule-binding domain, and the highly conserved carboxy terminus. In addition to the predicted fusion transcript, wild-type mRNA was also detected, indicating that splicing around the gene trap occurs. Activity of the lacZ marker was observed in all tissues throughout development and in all cells examined in the adult animal.