The nucleotide sequence of the phage lambda rex region consists of 1428 bp and codes for two genes, rexA and rexB. Hence the complete lambda immunity region codes for four genes and covers 2664 bp of sequence unique to lambda, as defined by the left and right boundaries of the imm434 region. Coordinate expression of both rexA and rexB, which are co-regulated with the cI repressor gene from promoters p rm and p re is responsible for the Rex phenotype, i.e. exclusion of a wide variety of superinfecting phage such as T4rII. The position of a third promoter, p lit, which overlaps the carboxy-terminal end of the rexA coding region, permits expression of rexB without rexA, from the resulting 470 nucleotide lit RNA. The lit transcript, therefore, must act as messenger for rexB in the noncoordinate expression of the rex genes that occurs late in lambda lytic infection. The coordinate and noncoordinate expression of rexB and rexA suggests a dual role for the very hydrophobic rexB protein. Studies of lambda early and late DNA replication implicate rexB as having auxiliary functions in both lysogenisation and lytic infection.