The rex gene of bacteriophage lambda is really two genes

Complementation tests among previously isolated rex- mutants of bacteriophage lambda reveal that the mutants comprise two complementation groups, designated rexA and rexB. Because rexB- mutants complement prm- mutants, but rexA- mutants do not, it appears that the rexA gene is coordinately controlled with the cI (repressor) gene under the direction of PRM promoter, but that some other promoter is capable of directing the expression of rexB.