Messenger RNAs (mRNAs) that contain premature translation termination codons 
(PTCs) are targeted for rapid degradation in all eukaryotes tested. The 
mechanisms of nonsense-mediated mRNA decay (NMD) have been described in 
considerable detail, but the biological roles of NMD in wild-type organisms are 
poorly understood. mRNAs of wild-type organisms known to be degraded by NMD 
("natural targets" of NMD) include by-products of regulated alternative 
splicing, out-of-frame mRNAs derived from unproductive gene rearrangements, 
cytoplasmic pre-mRNAs, endogenous retroviral and transposon RNAs, and mRNAs 
having upstream open reading frames or other unusual sequence features. NMD may 
function to eliminate aberrant PTC-containing mRNAs in order to protect cells 
from expression of potentially deleterious truncated proteins. Pseudogenes are 
nonfunctional genes or gene fragments that accumulate mutations through genetic 
drift. Such mutations will often introduce shifts of reading frame and/or PTCs, 
and mRNAs of expressed pseudogenes may thus be substrates of NMD. We demonstrate 
that mRNAs expressed from C. elegans pseudogenes are degraded by NMD and discuss 
possible implications for both mRNA surveillance and protein evolution. We 
describe an expressed pseudogene that encodes a small nucleolar RNA (snoRNA) 
within an intron and suggest this represents an evolutionary intermediate 
between snoRNA-encoding host genes that do or do not encode proteins.