PSMB5 and fragile X syndrome: Fragile X syndrome (FXS, OMIM #300624) is the most common monogenic cause of intellectual disability (ID) and belongs to the Fragile X-related disorders [1], a group of genetic conditions that includes also fragile X-associated tremor/ataxia syndrome (FXTAS, OMIM #300623) [2] and fragile X-associated primary ovarian insufficiency (FXPOI, OMIM #311360) [3].