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 a gRNA targeting EGFP, and a snRNA:U6:96Ac ( U6:3 ) promoter drives expression of a gRNA targeting GFP. 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.