A scientist used CRISPR-Cas9 to inactivate the pol gene for DNA polymerase in the porcine endogenous retrovirus (PERV)sequences of a pig embryo.The resulting pig embryo cells were then placed in proximity of human cells,but there was no infection of the human cells by the PERVs.Why were the PERVs no longer able to infect the human cells? 
Definitions:
Contemporary Psychology
The modern, ongoing study of the mind and behavior, incorporating the latest research and techniques.
Mental Processes
Internal cognitive operations that include thinking, perceiving, remembering, and making decisions.
Behavior
The actions or reactions of an object or organism, usually in relation to the environment.
Nature-nurture
A debate in psychology focused on the relative contributions of genetic inheritance and environmental factors to human development.
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