The RNA of the alphaviruses contains the genetic information for both the non-structural and the structural polypeptides. The former are contained in the initial two-thirds of the sequence (from the 5'-terminus) and are read directly from the virus particle RNA, but whether post-translational cleavage is involved or not is not known. The genes for the structural polypeptides are in the final one-third of the genome, and are probably not read directly but are transcribed selectively to produce a sub-genomic messenger RNA with only one initiation site. Translation produces the structural pooypeptides and post-translation cleavage does occur. However, the rate at which cleavage occurs is variable--cleavage to release the core polypeptide occurs while the nascent polypeptide is still attached to the ribosome, while cleavage to form two of the virus envelope glycoproteins occurs as the virus buds from the cell. The cleavage mechanism which leads to release of the core polypeptide also takes place in vitro and may be catalysed by the structural polypeptide itself.