A molecular biologist used a retroviral vector to introduce a gene coding for a certain human enzyme into mouse cells. One cell line was isolated that was able to make the human enzyme, but it had lost the ability to express an endogenous, normally expressed gene in the process. What is the best explanation for these results?

• A. The virus caused the mouse cells to become diseased.
• B. The virus had transferred a gene from one mouse cell to another.
• C. The virus was too small to carry the entire gene.
• D. The enzyme acted as a nuclease enzyme, cutting up mouse DNA.
• E. The virus inserted the gene encoding the human enzyme within the sequence of a normally expressed endogenous gene.