The T7 group of phages have a similar strategy of infection, which is largely determined by their transcription systems. The T7-like transcription system consists of a phage-encoded single-subunit RNA polymerase and a set of well-conserved T7-like promoters. Such a transcription system is a hallmark feature of the T7 group, and by this criterion, it's easy to tell whether a phage belongs to the T7 group, or it is just T7-like.

Several distantly related T7-like phages (phiKMV, Xp10 and P-SSP7) encode a T7-like RNAP, but they do not have a set of highly conserved T7-like promoters, suggesting that these phages have a different strategy of transcriptional control, therefore they do not belong to the T7 group. For more details see Nucleic Acids Res.,33(19):6172-87.

Sequence logos for promoters of eight T7 group phages: T7, phiA1122, T3, phiYeO3-12, SP6, K1-5, gh-1 and K11. In these sequence logos, the height of each letter is proportional to the frequency of that base at each position, and the height of the letter stack is the conservation in bits. All T7 group of phages have a set of highly conserved promoters with an information ranging from 31.0 to 36.7 bits.