The D protein engages in the head assembly of the wild-type λ phages. In the case where the mutant phage genome is smaller than 82% of the wild-type genomes, the assembly can be done without D protein. In addition, the assembly of virus particles can be done both in vivo and in vitro. One of the highlights of the system is that the proportion of fusion proteins to D proteins in phages can be regulated by the inhibitor tRNA activity of the host, which is particularly useful for displaying proteins that can cause damage to phage assembly.