a Strategy for conditional deletion of Bsx in septal neurons. b Representative in situ hybridisation for VG2 in control and Bsx cKO embryos at E13.5. c Representative expression of VG2 at two anterior-posterior levels of the septum (levels 2 and 5) in female adult control and Bsx cKO mice and quantification of VG2 neurons at five different anterior-posterior levels (anterior level 1 - posterior level 5). Control n = 6, Bsx cKO n = 5 mice at each level. 2-way RM ANOVA, Fisher’s LSD test, level 4 P = 0.009, level 5 P = 0.023. d Representative expression of CR at two rostro-caudal levels of the septum (levels 2 and 5) in female adult control and Bsx cKO mice and quantification of CR neurons at five different anterior-posterior levels. Control n = 4, Bsx cKO n = 3. 2-way RM ANOVA, Fisher’s LSD test. e Quantification of CB, p75 and PV neurons at five different rostro-caudal levels in female adult control and Bsx cKO mice. Control n = 4, Bsx cKO n = 3. 2-way RM ANOVA, Fisher’s LSD test. f Quantification of VG2, CR, CB, p75 and PV neurons at five different rostro-caudal levels in male adult control and Bsx cKO mice. VG2: n = 6 control, n = 4 Bsx cKO. CR, CB and PV, n = 3 mice of each genotype at each level except the following where n = 2: CB level 1 Bsx cKO, level 5 control and Bsx cKO and PV level 4 Bsx cKO. p75, control n = 4, Bsx cKO n = 3 except levels 1 and 2 Bsx cKO where n = 2. 2-way RM ANOVA, Fisher’s LSD test. VG2 level 4 P = 0.0006, level 5 P = 0.0013. All data show mean ± SEM. Source data are provided in Supplementary Data 1. Level 1: Bregma 1.18 mm; Level 2: Bregma 0.98 mm; Level 3: Bregma 0.74 mm; Level 4: Bregma 0.50 mm; Level 5: Bregma 0.26 mm. Scale bars: b 150 μm; c, d 100 μm.