Non-coding RNA genes include abundant and important RNAs such as transfer RNA (tRNA) and ribosomal RNA (rRNA); also RNAs such as snoRNAs, microRNAs, siRNAs, snRNAs, exRNAs, and piRNAs and the long noncoding RNAs (long ncRNAs). The number of ncRNAs in the human genome is unknown. However recent studies suggest the existence of thousands of ncRNAs. Since the function of newly identified ncRNAs has not been proved, it is possible that many are non-functional.