Our genome contains tens of thousands of long noncoding RNAs (lncRNAs), many of 
which are likely to have genetic regulatory functions. It has been proposed that 
lncRNA are organized into combinations of discrete functional domains, but the 
nature of these and their identification remain elusive. One class of sequence 
elements that is enriched in lncRNA is represented by transposable elements 
(TEs), repetitive mobile genetic sequences that have contributed widely to 
genome evolution through a process termed exaptation. Here, we link these two 
concepts by proposing that exonic TEs act as RNA domains that are essential for 
lncRNA function. We term such elements Repeat Insertion Domains of LncRNAs 
(RIDLs). A growing number of RIDLs have been experimentally defined, where 
TE-derived fragments of lncRNA act as RNA-, DNA-, and protein-binding domains. 
We propose that these reflect a more general phenomenon of exaptation during 
lncRNA evolution, where inserted TE sequences are repurposed as recognition 
sites for both protein and nucleic acids. We discuss a series of genomic screens 
that may be used in the future to systematically discover RIDLs. The RIDL 
hypothesis has the potential to explain how functional evolution can keep pace 
with the rapid gene evolution observed in lncRNA. More practically, TE maps may 
in the future be used to predict lncRNA function.