SL RNAs are small nuclear RNAs (snRNAs), which are functionally and structurally related to the snRNAs that make up the snRNPs ("snurps") involved in pre-mRNA splicing. The SL RNAs of nematodes carry a Spliced Leader sequence which is approximately 20 nucleotides long. They can be divided into two classes: "SL1-RNAs and "SL2-RNAs". SL1-RNAs transfer their 22 nt spliced leader sequence to 70% of the nuclear pre-mRNAs. The SL sequence of the SL2-RNAs are transferred during the processing of polycystronic pre-mRNAs transcribed from functional operons. That class of SL-RNA has only been found in C. elegans. A good review on SL-RNA structure and function was written some years ago by Nilsen (Annu. Rev. Microbiol.47:413, 1993). For more information about SL-RNAs and operons you can read the recent review by Blumenthal (Trend in Genetics, 11:132, 1995).