Mammalian-cell messenger RNAs (mRNAs) are generated in the nucleus from 
precursor RNAs (pre-mRNAs, which often contain one or more introns) that are 
complexed with an array of incompletely inventoried proteins. During their 
biogenesis, pre-mRNAs and their derivative mRNAs are subject to extensive 
cis-modifications. These modifications promote the binding of distinct 
polypeptides that mediate a diverse array of functions needed for mRNA 
metabolism, including nuclear export, inspection by the nonsense-mediated mRNA 
decay (NMD) quality-control machinery, and synthesis of the encoded protein 
product. Ribonucleoprotein complex (RNP) remodeling through the loss and gain of 
protein constituents before and after pre-mRNA splicing, during mRNA export, and 
within the cytoplasm facilitates NMD, ensuring integrity of the transcriptome. 
Here we review the mRNP rearrangements that culminate in detection and 
elimination of faulty transcripts by mammalian-cell NMD.