An Alternative String Theory in Twistor Space for N=4 Super-Yang-Mills
  In this letter, an alternative string theory in twistor space is proposed for describing perturbative N=4 super-Yang-Mills theory. Like the recent proposal of Witten, this string theory uses twistor worldsheet variables and has manifest spacetime superconformal invariance. However, in this proposal, tree-level super-Yang-Mills amplitudes come from open string tree amplitudes as opposed to coming from D-instanton contributions. 
Nathan Berkovits (IFT/UNESP, Sao Paulo)