  Special relativity forbids superluminal influences. Using only the
no-signaling principle and an assumption about the form of the Schmidt
decomposition, we show that for "any" allowed fidelity there is a "unique"
approximate qubit cloner which can be written explicitly. We introduce the
prime cloners whose fidelities have multiplicative property and show that the
fidelity of the prime cloners for the infinite copy limit is 1/2.
