FMR1 and fragile X syndrome: These observations indicate that gene dosage abnormalities of CYFIP1 can alter synaptic plasticity and function, and supports shared mechanisms between FXS and phenotypes associated with the loss of a functional copy of CYFIP1. The partial loss of CYFIP1 expression, when considered together with the restricted regional expression of CYFIP1, especially as compared to the ubiquitous expression of CYFIP2 and FMRP, could explain why the behavioural consequences of CYFIP1-deficiency are not necessarily as severe as what is observed with loss of FMRP.