Tor (target of rapamycin) pathway underlies a major signaling mechanism for controlling cell growth and proliferation.(1) Rheb (Ras homolog enriched in brain) is a small GTPase in the Tor pathway.(2-4) Similar to other small GTPases, Rheb cycles between a GTP-bound active state and a GDP-bound inactive state. TSC2 (tuberous sclerosis complex 2), a gene mutated in an autosomal dominant disease tuberous sclerosis, was shown to be the Rheb-GAP (GTPase activating protein).(5,6) However, a guanine nucleotide exchange factor (GEF) for Rheb had been missing. Human TCTP (translationally controlled tumor protein) has been implicated in cancer, but its function in vivo has not been clearly elucidated. Recently we reported a molecular genetic characterization of TCTP function in Drosophila.(7) Drosophila TCTP (dTCTP) displays GEF activity to Rheb and is essential for Rheb activation in organ growth. Thus, our study provides a tight linkage of dTCTP to the Rheb-TOR pathway. In this addendum, we will briefly overview our findings and discuss our perspectives for future research on TCTP.