CM13d3 is a novel helper phage engineered for multivalent phage display. CM13d3 lacks a functional gene III driving the display of pIII-fusion proteins expressed by the phagemid. CM13d3 is similar to pIII-defective helpers such as M13KO7d3, Hyperphage, Ex-Phage or Phaberge that have been shown to increase display of scFv by a factor up to 100-fold and more. CM13d3 is highly infective, making it de facto the best choice for building libraries.