Viscumin strongly inhibits protein synthesis by inactivating the 60 S ribosomal subunit. MAPK can also regulate translation by phosphorylating the S6 kinase in the large ribosomal subunit. Inactivation of antigen KI-67 leads to inhibition of ribosomal RNA synthesis. Resistance-conferring mutations of the 50S ribosomal subunit are rare. These structures consist of rough endoplasmic reticulum and associated ribosomal RNA. These are combined with ~50 ribosomal proteins to form ribosomal subunits. Ribosomes consist of two major components&colon; the small and large ribosomal subunits. Modifications to core ribosomal proteins may give rise to the formation of heterogeneous ribosomes. I is responsible for transcription of ribosomal RNA (rRNA) genes. Contacts across the two ribosomal subunits are known as intersubunit bridges. It forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA). Furthermore, it is associated with ribosomal RNA transcription. Some ribosomal peptides are subject to proteolysis. The group is defined primarily in terms of ribosomal RNA (rRNA) sequences. See nuclear import for more about the movement of the ribosomal proteins into the nucleus.