Glial fibrillary acidic protein (GFAP) is an intermediate filament protein found predominantly in astrocytes. This specificity has recommended the GFAP gene promoter for targeting transgene expression to astrocytes. Although both we [Brenner et al. J. Neurosci. 14:1030-1037, (1994)] and others [Mucke et al. New Biol. 3:465-474, (1991)] have reported astrocyte specificity for GFAP promoters, we demonstrate here that these DNA sequences can also direct activity in neurons. The pattern of neuronal activity varied with both the nature of the expressed sequence and the transgene insertion site. Specifically, neuronal expression was very high for a protective protein/cathepsin A minigene, moderate for lacZ and undetectable for GFP. These findings, coupled with a survey of the literature, recommend that investigators using GFAP-driven transgenes verify specificity for each line studied, using a detection system whose sensitivity is sufficient to detect a compromising level of misexpression.