Small nuclear RNA (snRNP, or 'snurps'), joins with proteins to form spliceosomes. The spliceosomes govern alternative splicing.

The background to this is that, in eukaryotes, most genes code for a protein in separated strings of DNA. This is because, of a total gene, coding bits (exons) are separated by non-coding bits (introns). The process called alternative splicing can produce many possible proteins from the gene parts because the proteins are put together in different ways. Alternative splicing produces alternative messenger RNAs, and these produce different proteins. Spliceosomes control the details of the splicing.