This represents a relatively well-conserved region near the C terminus of the tape measure protein of a lambda and related phage. The protein, which controls phage tail length, is typically about 1000 residues in length. Both low-complexity sequence and insertion/deletion events appear common in this family. Mutational studies suggest a ruler or template role in the determination of phage tail length. Similar behaviour is attributed to proteins from distantly related or unrelated families in other phage.