In the interview Montagnier gave to the French Journalist Djamel Tahi at the Pasteur Institute in July 1997 he was asked why he did not use immortal cell lines. His answer was that in such immortal cell lines such as MT2, MT4 (and thus HUT-78) "You have a mix of HIV and HTLV. It is a real soup."

But Montagnier may also have had a soup. There is a very good reason for this. Long before the AIDS era, retrovirologists knew that there was a significant difference between retroviruses and all other viruses. Unlike other viruses, retroviruses, especially in culture and under the conditions used by Montagnier, could arise even if the cells were not infected to begin with or if no retrovirus was introduced from outside. These retroviruses had become known as endogenous retroviruses.

Some of the best evidence for the existence of endogenous retroviruses comes from reproductive tissues including placenta and umbilical cord.. By using umbilical cord Montagnier had a soup of a different flavour, but still a soup.