By using an animal model, it is possible to learn more about CMV. The Guinea Pig model (GPCMV) of the virus is used because it shares many morphological and molecular similarities to CMV. The similarities between human and guinea pig placenta is a huge factor in why GPCMV is being used. The penetration of the virus through the placenta is very similar between the It is not an exact replica, but it is close enough that it is incredibly valuable to the research of CMV prevention. To study the genome of GPCMV, it must be modified and mutated to see how the genome responds to different types of changes. There are two methods used to modify CMV. The first, bacterial artificial chromosomes (BACs), is done by using E. coli to make mutant viruses. They are very stable and will usually not change over time. BACs allow for even the smallest change to the viral sequence. The second and the one that will be focused on, clusters of regularly interspaced short palindromic repeats or CRISPR/Cas9, uses RNA to cut sequences with Cas9 at desired locations. A difference between the two methods is that you cannot make lethal mutations with CRISPR because they occur during viral replication. Since BACs do not use viral replication, this restriction is not an issue.