a, RNA separated in RNA chip. b, c, IFN-β from MEFs transfected (for 18 h) with total (10 μg) RNA (b) or <200-nucleotide (nt) RNA (from 20 μg total RNA) (c). d, RIG-I (upper) and IRF-3 (lower) in RNase L-deficient MEFs and HT1080 cells, respectively, with <200-nt RNA (lanes 1–4) or wild-type MEFs (upper) or HT1080 cells (lower) infected with SeV (for 18 h) (lane 5). e–g, Levels of IFN-β from MEFs (wild-type, RIG-I-deficient, MDA5-deficient or IPS-1-deficient) incubated for 18 h with or without <200-nt RNA. Error bars, s.d.