Both 9-mth-old & 18-mth-old APN-KO spent significantly longer time in the margin area and shorter time in the center area (14.05% ± 2.28% in 9-mth-old WT mice vs 7.34% ± 2.19% in 9-mth-old APN-KO mice, Fig. 1a; 15.99% ± 1.43% in 18-mth-old WT mice vs 10.32% ± 1.41% in 18-mth-old APN-KO mice, mean ± S.E.M, p < 0.05, Fig. 1b), indicating that chronic APN deficiency in mice was associated with increased anxiety level. This evidence concerns the gene ANPEP and Anxiety.