Part of an organism's DNA is "non-coding DNA" sequences. They do not code for protein sequences. Some noncoding DNA is transcribed into non-coding RNA molecules, such as transfer RNA, ribosomal RNA, and regulatory RNAs. Other sequences are not transcribed at all, or give rise to RNA of unknown function. The amount of non-coding DNA varies greatly among species. For example, over 98% of the human genome is non-coding DNA, while only about 2% of a typical bacterial genome is non-coding DNA.