Mouse telomeric DNA sequence, Tel3.5: 5'-AGGG(T TAGGG)(3)-3', has the ability to form antiparallel tetraplex structure in the presence of Na(+). We examined the interaction between the antiparallel tetraplex structure of Tel3.5 and each of two single-stranded telomeric DNA-binding domains of mouse telomere binding protein Pot1, mPot1OB1 and mPot1OB2. The antiparallel tetraplex of Tel3.5 became unfolded upon the interaction with mPot1OB1. On the other hand, no significant structural change of the antiparallel tetraplex of Tel3.5 was observed upon the interaction with mPot1OB2. Considering that the antiparallel tetraplex inhibits telomerase-mediated telomere elongation, we conclude that the ability of mPot1OB1 to unfold the antiparallel tetraplex of the mouse telomeric DNA is required for telomerase-mediated telomere elongation.