Carbon nanotubes possess superior electrical and optical properties and hold great promise for electronic and biomedical applications1-10. The electronic properties of carbon nanotubes strongly depend on their chirality, with 1/3 being metallic1, 2/3 being semiconducting with band gaps inversely proportional to their diameter1. The lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of carbon nanotubes. Previous efforts in addressing the issue have resulted in significant progress in chirality resolved separation of synthetic mixtures, yielding nanotubes with predominant single chirality.11-14 However, separation processes are limited by their small scale, high cost, and short length (< 500 nm) of the resulting chirality-pure nanotubes, and therefore not viable for many, especially electronic device applications.11 Here, we demonstrate a general strategy for producing nanotubes of predefined chirality via combination of separation and synthesis. We show that purified single-chirality nanotube seeds can be significantly elongated through a catalyst-free chemical vapor deposition (CVD) process, producing horizontally aligned nanotube arrays with lengths of more than tens of micrometers on quartz substrates. Raman characterization confirms that the original chiralities of the nanotube seeds are preserved in the extended portion of the nanotubes, effectively achieving "chirality-controlled nanotube cloning", a goal that has eluded many previous efforts. The cloning process is found to be highly robust and can be carried out on various growth substrates, for different chiralities, and under several CVD growth conditions. The success of nanotube cloning should enable a range of fundamental studies and technological developments, especially the application of nanotubes for future beyond-silicon nanoelectronics.