Twistor origin of the superstring

After introducing a d=10 pure spinor λ α , the Virasoro constraint ∂x m ∂x m =0 can be replaced by the twistor-like constraint ∂x m ( γ m λ ) α = 0. Quantizing this twistor-like constraint leads to the pure spinor formalism for the superstring where the fermionic superspace variables θ α and their conjugate momenta come from the ghosts and antighosts of the twistor-like constraint