FMR1 and fragile X syndrome: In the absence of FMRP, LTD associated proteins are constitutively and highly expressed, and this makes a selective amplification of mGluR5 mediated LTD in the hippocampus of Fmr1 knockout (KO) mice, an animal model of fragile X syndrome (Huber et al., 2000; Bear et al., 2004; Waung and Huber, 2009; Krueger et al., 2011).