In the mouse model, we cannot exclude a role for phenotypic complementation of IN function through co-packaging of TP- Gag-Pol with full-length Gag-Pol expressed by ERVs in tumor cells, specifically transcomplementation by endogenous Gag-Pol proteins from either the Emv2 [22, 78] or the xenotropic MLV Bxv-1 loci. This evidence concerns the gene CD44 and neoplasm.