A partial map of λ phage. The genes and regulatory sites involved in establishing the lytic and lysogenic pathways are shown. Transcription begins at two opposing promoters, P_L and P_R, to produce the “immediate early” transcripts, which encode the N and Cro proteins. Two weak promoters, P_RM and P_RE, drive transcription of the cI gene, which encodes the λ repressor (cI). Overlapping the P_L, P_RM, and P_R promoters are two operator sites, O_L and O_R, each containing three binding sites for the λ repressor and Cro.