(A) Representative images of c-Fos staining in the CA3 subfield of the hippocampus, left, are home-cage controls and right are images from animals exposed to novel environments. Upper is WT and lower in Df(16)A/+. (B) Quantification of the c-Fos expression in CA3. In the home cage mutant and WT animals displayed similar levels of positive cells (7.9 ± 2.6 pyramidal cells per section in WT, n = 5 and 6.0 ± 2.0 in Df(16)A+/−, n = 5, p = 0.59), whereas activation of CA3 by exposure to the novel environment was weaker in Df(16) A+/− mice than WT controls (52.1 ± 4.9 pyramidal cells per section in WT, n = 9, and 38.7 ± 2.6 in Df(16)A+/−, n = 10, p = 0.023). (C) Quantification of the c-Fos expression in CA1. In both the home cage condition and after novel environment exploration the number of c-Fos positive pyramidal neurons showed no significant differences between genotypes (Home cage: 3.8 ± 1.8 pyramidal cells per section in WT, n = 5, and 2.6 ± 1.3 in Df(16)A+/−, n = 5, p = 0.60. Novel environment: 156.1 ± 16.1 pyramidal cells per section in WT, n = 9, and 133.6 ± 15.7 in Df(16)A+/−, n = 10, p = 0.33). (D) Automated cell counts of c-Fos positive pyramidal neurons in CA1 (left) and CA3 (right) gave similar results to those gained using visual inspection. (CA1: (117.7 ± 23.1 pyramidal cells per section in WT, n = 9, and 91.7 ± 21.5 in Df(16)A+/−, n = 10, p = 0.42. CA3: 42.7 ± 5.7 pyramidal cells per section in WT, n = 9, and 28.6 ± 3.8 in Df(16)A+/−, n = 10, p = 0.049). (E) Quantification of the c-Fos expression in the dentate gyrus. Novel environment exposure induced robust c-Fos expression in the suprapyramidal blade of the dentate. This increase was comparable between genotypes. Likewise no effect of genotype was observed in the home cage controls or in the infrapyramidal blade following exploration (Home cage: Infrapyramidal blade: 12.2 ± 2.5 granule cells per section in WT, n = 5, and 14.1 ± 3.1 in Df(16)A+/−, n = 5, p = 0.65. Suprapyramidal blade: 12.5 ± 1.7 in WT, n = 5, and 13.2 ± 1.6 in Df(16)A+/−, n = 5, p = 0.77. Novel environment: Infrapyramidal blade: 9.5 ± 0.9 granule cells per section in WT, n = 9, and 8.6 ± 1.1 in Df(16)A+/−, n = 10, p = 0.54. Suprapyramidal blade: 27.2 ± 1.4 in WT, n = 9, and 26.2 ± 2.0 in Df(16)A+/−, n = 10, p = 0.70).