Twistor Origin of the Superstring
  After introducing a d=10 pure spinor $\lambda^\alpha$, the Virasoro constraint $\partial x^m \partial x_m =0$ can be replaced by the twistor-like constraint $\partial x^m (\gamma_m \lambda)_\alpha=0$. Quantizing this twistor-like constraint leads to the pure spinor formalism for the superstring where the fermionic superspace variables $\theta^\alpha$ and their conjugate momenta come from the ghosts and antighosts of the twistor-like constraint. 
Nathan Berkovits (ICTP-SAIFR/IFT-UNESP, Sao Paulo)
hep-th