FMR1 and fragile X syndrome: Therefore, it is possible that the disrupted mechanism driving this genotype difference is due to a separate mechanism than the hypothesized deficiency in pruning thought to contribute to Fmr1 KO mice and FXS patients spine alterations (Kazdoba et al., 2014; Galvez and Greenough, 2005; McKinney et al., 2005; Irwin et al., 2001).