Transcripts of RNA polymerase II undergo several processing events in the cell nucleus before they can be exported to the cytoplasm as functional mRNAs that code for proteins. This complexity of post-transcriptional events renders eukaryotic RNA processing prone to errors that may be harmful or even fatal, and requires mechanisms that control the quality of the processed transcript. The elimination of transcripts harboring premature termination codons (PTCs) is particularly important, because they might code for defective and harmful proteins. Two nuclear mechanisms through which PTCs are eliminated by alternative splicing are addressed. One is the nonsense-mediated altered splicing (NAS), and the other is the stop codon mediated suppression of splicing (SOS). A question pertaining both mechanisms is how a protein reading frame can be recognized in the cell nucleus prior to splicing? A speculative model, based on the structure of the supraspliceosome, is presented to explain this enigma.