The FXR gene family consists of FMR1, FXR1 and FXR2. Loss of FMR1 function causes fragile X syndrome in humans but the contribution of related genes to disease is largely unknown. Coffee, Jr et al. show that FMR1 function in Drosophila neurons is distinct from other gene family members, as neither FXR1 nor FXR2 rescue FMR1 deficiency in neurons. However, expression of any of the three human proteins reverses deficiency effects on spermatogenesis. Redundancies in FXR gene functions are tissue specific in the fly.