The telomeric repeat amplification protocol (TRAP) is commonly used to evaluate telomerase activity in tissues or cell extracts and also to determine the inhibitory properties of small molecules against telomerase. The recent discovery of G-quadruplex ligands as potent telomerase inhibitors prompted us to examine the accuracy of TRAP to be used to screen such class of molecules. Because of the specific feature of the TS primer, TRAP only allows the detection of G-quadruplex-induced telomerase inhibition after the synthesis of four TTAGGG repeats by telomerase and may thus lead to misinterpretations during screening assays. We have developed a TRAP-G4 assay that will allow the unambiguous detection of the inhibitory properties of a G-quadruplex ligand on telomerase activity and is able to discriminate them from other telomerase inhibitors.