Unfortunately, major findings of the outlined in vitro studies could not be reproduced by independent groups, and others raised concerns that these findings might mainly reflect contamination or technical artifacts [151] . In an independent study, a large number of sera of PBC patients and healthy controls did not show reactivity against MMTV encoded protein, and no detectable immunohistochemical or molecular evidence for MMTV was found in liver specimens or peripheral blood lymphocytes [152] . It was also speculated that beneficial effects of antiretroviral therapy could be partly explained by anti-apoptotic properties of nucleoside analogs [151, 153] . Furthermore, mechanisms by which human betaretrovirus would enter human cholangiocytes are also not identified.