890 bp of coding sequence of the intronless Nap1l2 gene between two HincII restriction endonuclease recognition sites were replaced, in-frame, by a nuclear-localized beta-galactosidase (lacZ) reporter gene and a neomycin resistance gene. The insertion of these cassettes disrupts the reading frame of the remainder of the Nap1l2 gene, so that the resulting fusion protein includes only 5 N-terminal amino acids of the NAP1L2 protein. Beta-galactosidase expression in developing chimeric embryos was first observed by X-gal staining in the tip of the tail at embryonic day (E) 9.5, and by E10.5 throughout the neural tube. By E12.5 the neural tube, ganglia and some upper and lower thoracic muscles were labeled. Histologic brain sections showed strong lacZ expression in the rhombencephalon and spinal cord.