Reduction of off-target RNA interference toxicity

A nucleic acid encoding an artificial primary miRNA transcript (pri-miRNA) consisting of, in order of position, a 5′-flanking region, a first siRNA region, a loop region, a second siRNA region, and a 3′-flanking region, wherein the 5′-flanking region comprises:

• (a) a 5′-joining sequence contiguously linked to the first siRNA region, wherein the 5′-joining sequence consists of 5-7 nucleotides;
• (b) a 5′-bulge sequence positioned upstream from the 5′-joining sequence, wherein the 5′-bulge sequence consists of 1-10 nucleotides;
• (c) a 5′-spacer sequence positioned upstream from the 5′-bulge sequence, wherein the 5′-spacer sequence consists of 10-12 nucleotides;
• and
• (d) a 5′-upstream sequence positioned upstream from the 5′-spacer sequence, wherein the 5′-upstream sequence is about 30-2000 nucleotides in length;
• and wherein the 3′-flanking region comprises:
• (e) a 3′-joining sequence contiguously linked to the first siRNA region, wherein the 3′-joining sequence consists of 5-7 nucleotides;
• (f) a 3′-bulge sequence positioned downstream from the 3′-joining sequence;
• wherein the 3′-bulge sequence consists of 1-10 nucleotides;
• (g) a 3′-spacer sequence positioned downstream from the 3′-bulge sequence, wherein the 3′-spacer sequence consists of 10-12 nucleotides;
• and
• (h) a 3′-downstream sequence positioned downstream from the 3′-spacer sequence, wherein the 3′-downstream sequence is about 30-2000 nucleotides in length.