During pregnancy in viviparous mammals (all mammals except Monotremes), endogenous retroviruses (ERVs) are activated and produced in high quantities during the implantation of the embryo.
Durante o embarazo nos mamíferos vivíparos (todos menos os Monotremas), os retrovirus endóxenos (ERVs) inseridos no xenoma son activados e prodúcense en grandes cantidades durante a implantación do embrión.