Three experiments reveal key features of gene silencing. (a) Fire and Mello demonstrated that dsRNA copies can selectively silence genes in C. elegans. (b) Jorgensen discovered that a transgene can silence an endogenous petunia gene necessary for floral color. (c) Baulcombe showed that plants with a copy of a viral transgene were resistant to viral infection and produced siRNAs complementary to the viral genome.