Definition for Cro protein
From Biology Forums Dictionary
Phage Cro Protein is involved in transcription, the process of copying DNA's genetic information into RNA so that it can be made into proteins, and DNA's instructions can be carried out within the cell. The University of Nebraska has a good animation that describes the process of transcription in more detail (although the animation describes transcription in a plant cell, the concept is essentially the same).
Cro is a negative regulator. That is, it works to repress the transcription of other proteins (primarily, lambda repressor) by binding preferentially to the operator site that partially covers lambda repressor's promoter region. The mechanism by which Cro works, alongside lamda repressor, is perhaps the most well known (Degnan et al, 2007). Additionally, other phages utilize essentially the same mechanism. The regions in which Cro and lambda repressor are found are conserved throughout most of the other phages.
It has even been suggested that the nonspecific binding by Cro protein can also play an affect within the cell. In fact, for relatively high concentrations of Cro, upwards of half of the protein is bound nonspecifically. Bakk and Metzler suggest that the nonspecific binding of Cro and lambda repressor is important for maintaing the stability of the genetic switch.