Modifications at the C-2' ribose position are also common, with 2'-O-methyl and 2'-deoxy modifications as the most highly represented and 2'-fluoro and 2'-MOE somewhat less. The prevalence of thymidine (deoxythymidine) at sequence lengths of 21 and 42 is because thymidine at the 3' end of artificial siRNAs protects them from exonucleases. Artificial siRNAs are predominantly 21-mers, and since they are double-stranded, they are highly represented at both 21 and 42 nucleotides. Phosphorothioate is the most common phosphate modification for the sugar-phosphate backbone, although phosphorodithioate also occurs. Altered ribose stereochemistry, such as L-ribose in place of D-ribose, is a relatively rare modification, as are replacement backbones, such as PMO, GNA, and PNA.