Different types of post-transcriptional transgene-induced gene silencing have been shown to occur in Neurospora (quelling) and plants (co-suppression). Common mechanistic features include triggering by duplicated coding sequences, simultaneous silencing of the transgene and endogenous genes, reversibility of silencing, and post-transcriptional reduction of gene-specific mRNA. These shared features suggest that this type of gene silencing may have evolved from a common ancestral mechanism designed to protect genome integrity. Understanding the mechanisms involved may be particularly useful in biotechnological applications aimed at enhancing or avoiding gene silencing in transgenic plants and fungi.