We have produced nine transgenic mice lines carrying either 5 kbp or 407 bp of the 5' flanking sequence of the rat light neurofilament gene linked to the chloramphenicol acetyltransferase (CAT) structural gene. With the 5kb light neurofilament 5' flanking region governing the expression of CAT, reporter gene activity was detected not only in brain but also in the eye lens and skeletal muscle, yet not in other tissues. With the 407 bp construct, reporter gene activity was detected only in the brain, although expression was approximately one tenth of that found with the 5 kb 5' region. These results, together with earlier observations, indicate that the sequence -407 to -292 of the proximal promoter region for the light neurofilament gene or sequence +15 to +75 bp after the transcription initiation site is crucial for brain-specific expression of a fusion gene in transgenic mice.