Viruses are made out of nucleic acids, either RNA or DNA, but never both. A protein coat (capside) or shell covers and protects the nucleic acid core in all viruses. A lipid envelope or membrane surrounds the nucleocapsid core of certain viruses. Viral proteins may also be found associated with the nucleocapsid core or enclosed within membranes. They can be classified into several groups based on their chemical composition and function. RNA virus cores contain ribonucleoproteins that consist of an RNA molecule attached to a nucleus-specific protein called RNP complexing with other viral proteins. The ribonucleoprotein core is surrounded by a lipoprotein layer derived from the virus membrane. Virus-associated proteins are required for replication and transcription of the genome and assembly into virions. Some remain bound to the genomic material after infection is established while others are removed during maturation of the virus particle. Proteins are also released from infected cells in large quantities during lysis to provide nutrients for viral replication. These proteins act as enzymes, providing new surface receptors for infection of additional cells. Some become incorporated into new virions and some are degraded by proteases present in infected cells. DNA viruses contain protein capsids that enclose their genetic material. The capsid is composed of several hundred copies of a single protein. Certain viruses also contain a second type of protein called scaffolding that provides structure for assembling the capsid.