Immediate early genes. Very strong promoters of gene expression. No protein is expressed. The gene binds RNA polymerase to start transcription of the adjacent early and late genes. The IE gene promoter from human cytomegalovirus is often ligated into DNA vaccines just upstream of foreign protective gene.

Early genes. These encode the proteins involved in stimulating virus replication by many means. eg synthesis of more viral DNA by the rolling circle mechanism. Early genes are the oncogenes of Mareks disease virus of chickens and Epstein-Barr virus of man (see table).

Late genes. These encode the virus structural proteins. Large inclusion bodies of capsid are assembled in the nucleus to cause cytomegaly- the formation of large cells with large owl's eye inclusion bodies. The envelope buds through the nuclear membrane. The masses of new virus then lyse the cell or fuse cells.