A snRNA:U6:96Ac ( U6:3 ) promoter drives expression of a quadruple-gRNA comprising a gRNA targeting comt, a gRNA targeting Nsf2, a second gRNA targeting Nsf2, and a second gRNA targeting comt, each separated by a tRNA-Gln (Q). 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.