A dual-gRNA in which the CR7T promoter (a pol III promoter reported to be potent in both the soma and the germline) drives expression of one gRNA targeting gnu, and the snRNA:U6:96Ac ( U6:3 ) promoter drives expression of a second gRNA targeting gnu. The 2.1 gRNA scaffold (which includes an additional extension of the second stem loop compared to the (F+E) gRNA scaffold described in FBrf0241409) is used.