Some pieces of Ancient technology can only be activated by a user with a specific genetic makeup, one who possesses a particular Ancient gene. Of SG-1, only Jack O'Neill possesses the gene, which may have accounted for his ability to access the knowledge of the Ancient repository which did not readily respond to Teal'c or Daniel. Using the knowledge of the repository, O'Neill was drawn to Praclarush Taonas and to the Ancient outpost in Antarctica where a chair of Ancient design could only be activated by someone who possessed the gene. O'Neill activated both devices with his mind, and the Ancient defense weapon in Antarctica defended Earth from Anubis. When SG-1 visited the world where Maybourne ruled as King Arkhan, they discovered an abandoned Ancient spacecraft which responded to the Ancient gene in O'Neill, allowing him to fly it through thought alone. The ship also contained a time travel device which O'Neill was later able to activate to carry the team back to ancient Egypt.
An Ancient communication stone discovered on P3R-233 responded to the Ancient gene in O'Neill, and a similar stone found in an archaeological dig in Egypt and sold at a garage sale responded to Joe Spencer, the barber in Indiana who bought the stone and who was also discovered to have the gene. For seven years the two stones allowed for a sort of psychic connection between O'Neill and Spencer.
Although it was originally believed that the stones could only be used by someone with the Ancient gene, it was later determined that the stones, like a lot of Ancient technology, need only be initialized by someone with the gene, as O'Neill had done, after which they could be used by anyone. The stones that had been set for O'Neill and Spencer were later reset, and Daniel and Vala were able to use them in conjunction with the communication terminal to open a psychic link to the Ori galaxy. In the case of the Ancient repository, the two repositories encountered by SG-1 were depleted or destroyed after O'Neill had interfaced with them, and consequently it had not been proven that the Ancient gene is required to access that technology. However, the repository in Merlin's cave did respond to Daniel after Merlin had initialized it, and Merlin's consciousness was downloaded into Daniel.
Cross Reference: Ancients, Ancient Communication Stone, Ancient Communication Terminal, Ancient Defense Weapon, Ancient Repository, Ancient Spacecraft, Ancient Time Travel Device, Jack O'Neill, Joe Spencer
Episode Reference: Lost City, It's Good to Be King, Citizen Joe, Moebius, Avalon