Basal neural precursor cell proliferation in the hippocampal dentate gyrus SGZ and GL showing statistically significant decreases in Npas3^-/- (n = 8) adult mice relative to wild-type littermate (n = 8) and Npas1^-/- (n = 11) animals (*, P < 0.0001, Student's t test). (A) Mice received daily i.p. injection of BrdUrd (50 μg/g of body weight) for 12 days and representative immunohistochemical staining of incorporated BrdUrd on day 13 is shown in hippocampal dentate gyrus of wild-type, Npas3^-/-, and Npas1^-/- adult mouse brain. Hematoxylin/eosin staining showed markedly reduced thickness of the dentate gyrus granular layer in Npas3^-/- adult mice relative to wild-type and Npas1^-/- adult mice. (B) Average numbers of BrdUrd-positive cells per section within the dentate gyrus were 74 ± 8 in wild-type mice, 12 ± 3 in Npas3^-/- mice, and 73 ± 9 in Npas1^-/- mice