IL17A and Down syndrome: These controversial findings are also reported in multiple brain diseases including stroke, Down syndrome, and post-traumatic stress disorder (PTSD), i.e., (1) anti-proliferative effects by IL-17A in Down syndrome and PTSD vs. inhibitory or no effects on cell proliferation in stroke, and (2) inhibition of neurogenesis in PTSD vs. inhibition or promotion of neurogenesis in stroke (Lin et al., 2016; Rueda et al., 2018; Sun et al., 2020a,b; Willinger and Turgeman, 2022).