We analyzed the structural properties of mouse telomeric DNA sequence, Tel3.5: 5'-AGGG(TTAGG G)3-3', and nontelomeric DNA sequence, T22: 5'-T22-3', and examined the interaction with a single-stranded telomeric DNA-binding domain of mouse telomeric DNA-binding protein Pot1 (mPot1DBD). T22 did not form any higher-order structure, but Tel3.5 formed antiparallel tetraplex structure in the presence of Na(+). The antiparallel tetraplex of Tel3.5 became unfolded upon the interaction with mPot1DBD. Considering that the antiparallel tetraplex is known to inhibit telomerase-mediated telomere elongation, we conclude that the ability of Pot1 to unfold the antiparallel tetraplex of the telomeric DNA is required for regulation of telomerase-mediated telomere elongation.