The host RNA polymerase initially transcribes two genes, which are called the immediate early genes (N and cro). The transition to the next stage of expression is controlled by preventing termination at the ends of the immediate early genes, with the result that the delayed early genes are expressed. The antitermination protein pN acts specifically on the immediate early transcription units. Later during infection, another antitermination protein pQ acts specifically on the late transcription unit, to allow its transcription to continue past a termination sequence.