Life on Earth is thought to have begun in an RNA world more than 3 billion years ago. Today, RNA continues to be of vital importance to cellular function. It can both catalyze biochemical reactions (the ribosome is a ribozyme) and transmit genetic information. RNA can also guide protein enzymes to specific sites in DNA or RNA, as occurs in CRISPR-Cas function.<br><br>RNAs have evolved to work closely with proteins, forming ribonucleoprotein complexes (RNPs). Many nanomachines of the cell are RNPs, including the ribosome, spliceosome, telomerase, and signal recognition particle. RNAs regulate transcription both directly, as is the case for E. coli 6S RNA, and indirectly, as in X chromosome inactivation in mammals by Xist. RNA-binding proteins autoregulate their synthesis by binding to their mRNAs and attenuating transcription, splicing, or translation.<br><br>The early steps of embryo development are regulated by maternal mRNA translation since zygotic transcription begins only after several cell divisions. Small non-coding RNAs contained in sperm can convey paternal epigenetic information to the developing zygote. Viral RNAs invade cells to wrest control of the genetic program but can be silenced by cellular RNAs. Many diseases are caused by defects in RNA synthesis and processing, and synthetic RNAs are currently used to treat a few of these diseases.