Scientists have discovered that some tiny segments of RNA thought to be junk instead have a functional role in suppressing production of certain messenger RNAs and appear to help cells respond to oxidative stress. 

The segments in question are introns, short sequences of a subset of transfer RNAs, the RNA molecules that help guide assembly of amino acid chains during protein construction. These introns have historically been thought to be useless to cells because they must be clipped off before tRNA can do its job.