A U6 promoter drives expression of 2 sgRNAs (5'-tggatatatctttcggctag-agg-3' and 5'-agaccaaaacctgtggggca-agg-3'), both of which target intron 2 of the L-RC isoform (the two target sites flank the location of the opus{}Mohr insertion that is present in a number of dominant L mutant chromosomes).