This protocol details how to create humanized mouse models from NSG mice. Four different variants of humanized mice can be generated based on whether or not the native thymus is retained or if a human thymus piece is transplanted. Which type of humanized mouse is desired depends on the goals for the experiment. See our protocols on NSG mouse thymectomy, human CD34+ cell isolation, and human fetal thymus preparation for more details on some of the steps in this protocol (“Thymectomy procedure to remove native thymus of NSG mice”, “Human CD34+ cell isolation from fetal liver, and fetal thymus preparation”, “CD34+ isolation from human bone marrow”).