In some DNA molecules, replication is terminated whenever two replication forks meet. In others, specific termination sequences (called Ter sites) block further replication. A termination protein, called Tus in E. coli, binds to these sequences, creating a Tus-Ter complex that blocks the movement of helicase, thus stalling the replication fork and preventing further DNA replication. Each Tus-Ter complex blocks a replication fork moving in one direction, but not the other.