FMR1 and fragile X syndrome: Moreover, both FXS individuals and Fmr1 KO mice have altered brainstem-mediated acoustic reflexive behaviors, such as pre-pulse inhibition (PPI) of acoustic startle (McCullagh et al., 2020b; Frankland et al., 2004; Hessl et al., 2009; Koch, 1999; Fendt et al., 2001).