Nonsense-mediated mRNA decay (NMD) is a cellular RNA surveillance system that recognizes transcripts with premature termination codons and degrades them. We previously discovered large numbers of natural alternative splice forms that appear to be targets for NMD, and we speculated that this might be a mode of gene regulation which we termed RUST (regulated unproductive splicing and translation). This seems to be confirmed by our finding that all conserved members of the SR family of splice regulators have an unproductive alternative mRNA isoform targeted for NMD. Strikingly, the splice pattern for each is conserved in mouse and always associated with an ultraconserved or highly-conserved region of ~100 or more nucleotides of perfect identity between human and mouse. Remarkably, this seems to have evolved independently in every one of the genes, suggesting that this is a natural mode of regulation.