Mapping of the CYFIP1 gene encoding an FMRP interacting protein within the PWS candidate region is of particular relevance for the analysis of the Prader-Willi-like phenotype observed in fragile X patients with full mutations in FMR1. The significance of the overlapping clinical features between PWS and fragile X syndrome remains uncertain, and it is very difficult to determine whether there is any intrinsic link underlying these overlapping clinical features solely from a clinical standpoint. Here, CYFIP1 is linked to Prader-Willi syndrome.