ShortCut® RNase III, used with its manganese-containing reaction buffer, converts long double-stranded RNA into a heterogeneous mix of short (18–25 bp) interfering RNAs (siRNA) suitable for RNA interference in mammalian cells (1–3). 1.5 units (1 µl) of ShortCut RNase III is sufficient to convert 1 µg of dsRNA into siRNA suitable for RNA interference in mammalian cells.